Multi-Touch Systems that I Have Known and Loved

 

Bill Buxton
Microsoft Research
Original: Jan. 12, 2007
Version:  Auguest 18, 2008

Keywords / Search Terms

Multi-touch, multitouch, input, interaction, touch screen, touch tablet, multi-finger input, multi-hand input, bi-manual input, two-handed input, multi-person input, interactive surfaces, soft machine, hand gesture, gesture recognition .

Preamble

Since the announcement of the iPhone, an especially large number of people have asked me about multi-touch.  The reason is largely because they know that I have been involved in the topic for a number of years.  The problem is, I can't take the time to give a detailed reply to each question.  So I have done the next best thing (I hope).  That is, start compiling my would-be answer in this document.  The assumption is that ultimately it is less work to give one reasonable answer than many unsatisfactory ones.

Multi-touch technologies have a long history.  To put it in perspective, the original work undertaken by my team was done in 1984, the same year that the first Macintosh computer was released, and we were not the first.  Furthermore, there was a significant body of prior art on which multi-touch was built.

The following is a very brief and incomplete summary of some of the landmark examples that I have been involved with and/or known about and encountered over the years.  It is incomplete and a work in progress (so if you come back a second time, chances are there will be more and better information).I apologize to those that I have missed.  I have erred on the side of timeliness vs thoroughness.  Other work can be found in the references to the papers that I do include.

Please do not be shy in terms of sending me photos, updates, etc.  I will do my best to integrate them.

For more background on input, see also the incomplete draft manuscript for my book on input tools, theories and techniques:

                http://www.billbuxton.com/inputManuscript.html

For more background on input devices, including touch screens and tablets, see my directory at:

                http://www.billbuxton.com/InputSources.html

I hope this helps.

Some Dogma

There is a lot of confusion around touch technologies, and despite a 25 year history, very little information or experience with multi-touch interaction.  I have three comments to set up what is to follow:

1.       Remember that it took 30 years between when the mouse was invented by Engelbart and English in 1965 to when it became ubiquitous, on the release of Windows 95.  Yes, it was released commercially on the Xerox Star and PERQ workstations in 1982, and I used my first one in 1972 at the National Research Council of Canada.  But statistically, that doesn’t matter.  It took 30 years to hit the tipping point.  So, by that measure, multi-touch technologies have 5 years to go before they fall behind.

2.       Keep in mind one of my primary axioms: Everything is best for something and worst for something else.  The trick is knowing what is what, for what, when, for whom, where, and most importantly, why.  Those who try the replace the mouse play a fool’s game.  The mouse is great for many things.  Just not everything.  The challenge with new input is to find devices that work together, simultaneously with the mouse (such as in the other hand), or things that are strong where the mouse is weak, thereby complimenting it.

3.       To significantly improve a product by a given amount, it probably takes about two more orders of magnitude of cost, time and effort to improve the display as to get the same amount of improvement on input.  Why?  Because we are ocular centric, and displays are therefore much more mature.  Input is still primitive, and wide open for improvement.  So it is a good thing that you are looking at this stuff.  What took you so long?

Some Framing

I don’t have time to write a treatise, tutorial or history.  What I can do is warn you about a few traps that seem to cloud a lot of thinking and discussion around this stuff.  The approach that I will take is to draw some distinctions that I see as meaningful and relevant.  These are largely in the form of contrasts:

• Touch-tablets vs Touch screens: In some ways these are two extremes of a continuum.  If, for example, you have paper graphics on your tablet, is that a display (albeit more-or-less static) or not? What if the “display” on the touch tablet is a tactile display rather than visual?  There are similarities, but there are real differences between touch-sensitive display surfaces, vs touch pads or tablets. It is a difference of directness. If you touch exactly where the thing you are interacting with is, let’s call it a touch screen or touch display.  If your hand is touching a surface that is not overlaid on the screen, let's call it a touch tablet or touch pad.

• Discrete vs Continuous: The nature of interaction with multi-touch input is highly dependent on the nature of discrete vs continuous actions supported.  Many conventional touch-screen interfaces are based discrete items such as pushing so-called "light buttons", for example.  An example of a multi-touch interface using such discrete actions would be using a soft graphical QWERTY keyboard, where one finger holds the shift key and another pushes the key for the upper-case character that one wants to enter.  An example of two fingers doing a coordinated continuous action would be where they are stretching the diagonally opposed corners of a rectangle, for example. Between the two is a continuous/discrete situation, such as where one emulates a mouse, for example, using one finger for indicating continuous position, and other fingers, when in contact, indicate mouse button pushes, for example.

• Degrees of Freedom: The richness of interaction is highly related to the richness/numbers of degrees of freedom (DOF), and in particular, continuous degrees of freedom, supported by the technology.  The conventional GUI is largely based on moving around a single 2D cursor, using a mouse, for example.  This results in 2DOF.  If I am sensing the location of two fingers, I have 4DOF, and so on.  When used appropriately, these technologies offer the potential to begin to capture the type of richness of input that we encounter in the everyday world, and do so in a manner that exploits the everyday skills that we have acquired living in it.  This point is tightly related to the previous one.

• Size matters:  Size largely determines what muscle groups are used, how many fingers/hands can be active on the surface, and what types of gestures are suited for the device.

• Orientation Matters - Horizontal vs Vertical: Large touch surfaces have traditionally had problems because they could only sense one point of contact.  So, if you rest your hand on the surface, as well as the finger that you want to point with, you confuse the poor thing.  This tends not to occur with vertically mounted surfaces.  Hence large electronic whiteboards frequently use single touch sensing technologies without a problem.

• There is more to touch-sensing than contact and position:  Historically, most touch sensitive devices only report that the surface has been touched, and where.  This is true for both single and multi touch devices.  However, there are other aspects of touch that have been exploited in some systems, and have the potential to enrich the user experience:

  1. Degree of touch / pressure sensitivity: A touch surfaces that that can independently and continuously sense the degree of contact for each toouch point has a far higher potential for rich interaction.  Note that I use “degree of contact” rather than pressure since frequently/usually, what passes for pressure is actually a side effect – as you push harder, your finger tip spreads wider over the point of contact, and what is actually sensed is amount/area of contact, not pressure, per se.  Either is richer than just binary touch/no touch, but there are even subtle differences in the affordances of pressure vs degree.

  2. Angle of approach:  A few systems have demonstrated the ability to sense the angle that the finger relative to the screen surface.  See, for example, McAvinney's Sensor Frame, below.  In effect, this lgives the finger the capability to function more-or-less as a virtual joystick at the point of contact, for example.  It also lets the finger specify a vector that can be projected into the virtual 3D space behind the screen from the point of contact - something that could be relevant in games or 3D applications.

  3. Force vectors:  Unlike a mouse, once in contact with the screen, the user can exploit the friction between the finger and the screen in order to apply various force vectors.  For example, without moving the finger, one can apply a force along any vector parallel to the screen surface, including a rotational one.  These techniques were described as early as 1978 [Herot, C. & Weinzapfel, G. (1978). One-Point Touch Input of Vector Information from Computer Displays, Computer Graphics, 12(3), 210-216.] and again five years later [Minsky, M. (1984). Manipulating Simulated Objects with Real-World Gestures Using a Force and Position Sensitive Screen, Computer Graphics, 18(3), 195-203.].

  Such historical examples are important reminders that it is human capability, not technology, that should be front and centre in our considerations.  While making such capabilities accessible at reasonable costs may be a challenge, it is worth remembering further that the same thing was also said about multi-touch.  Furthermore, note that multi-touch dates from about the same time as these other touch innovations.

• Size matters II: The ability of to sense the size of the area being touched can be as important as the size of the touch surface.  See the Synaptics example, below, where the device can sense the difference between the touch of a finger (small) vs that of the cheek (large area), so that, for example, you can answer the phone by holding it to the cheek.

• Single-finger vs multi-finger: Although multi-touch has been known since at least 1982, the vast majority of touch surfaces deployed are single touch.  If you can only manipulate one point, regardless of with a mouse, touch screen, joystick, trackball, etc., you are restricted to the gestural vocabulary of a fruit fly.  We were given multiple limbs for a reason. It is nice to be able to take advantage of them.

• Multi-point vs multi-touch:  It is really important in thinking about the kinds of gestures and interactive techniques used if it is peculiar to the technology or not.  Many, if not most, of the so-called “multi-touch” techniques that I have seen, are actually “multi-point”.  Think of it this way: you don’t think of yourself of using a different technique in operating your laptop just because you are using the track pad on your laptop (a single-touch device) instead of your mouse.  Double clicking, dragging, or working pull-down menus, for example, are the same interaction technique, independent of whether a touch pad, trackball, mouse, joystick or touch screen are used.

• Multi-hand vs multi-finger:  For much of this space, the control can not only come from different fingers or different devices, but different hands working on the same or different devices.  A lot of this depends on the scale of the input device.  Here is my analogy to explain this, again referring back to the traditional GUI. I can point at an icon with my mouse, click down, drag it, then release the button to drop it.  Or, I can point with my mouse, and use a foot pedal to do the clicking.  It is the same dragging technique, even though it is split over two limbs and two devices.  So a lot of the history here comes from a tradition that goes far beyond just multi-touch.

• Multi-person vs multi-touch:  If two points are being sensed, for example, it makes a huge difference if they are two fingers of the same hand from one user vs one finger from the right hand of each of two different users.  With most multi-touch techniques, you do not want two cursors, for example (despite that being one of the first thing people seem to do).  But with two people working on the same surface, this may be exactly what you do want.  And, insofar as multi-touch technologies are concerned, it may be valuable to be able to sense which person that touch comes from, such as can be done by the Diamond Touch system from MERL (see below).

• Points vs Gesture:  Much of the early relevant work, such as Krueger (see below) has to do with sensing the pose (and its dynamics) of the hand, for example, as well as position.  That means it goes way beyond the task of sensing multiple points.

• Stylus and/or finger:  Some people speak as if one must make a choice between stylus vs finger.  It certainly is the case that many stylus systems will not work with a finger, but many touch sensors work with a stylus or finger.  It need not be an either or question (although that might be the correct decision – it depends on the context and design).  But any user of the Palm Pilot knows that there is the potential to use either.  Each has its own strengths and weaknesses.  Just keep this in mind:  if the finger was the ultimate device, why didn’t Picasso and Rembrandt restrict themselves to finger painting? On the other hand, if you want to sense the temperature of water, your finger is a better tool than your pencil.

• Hands and fingers vs Objects:  The stylus is just one object that might be used in multi-point interaction. Some multi-point / multi-touch systems can not only sense various different objects on them, but what object it is, where it is, and what its orientation is.  See Andy Wilson’s work, below, for example.  And, the objects, stylus or otherwise, may or may not be used in conjunction and simultaneously with fingers.

• Different vs The Same:  When is something the same, different or obvious?  In one way, the answer depends on if you are a user, programmer, scientist or lawyer.  From the perspective of the user interface literature, I can make three points that would be known and assumed by anyone skilled in the art:

  1. Device-Independent Graphics: This states that the same technique implemented with an alternative input device is still the same technique. For example, you can work your GUI with a stylus, touch screen, mouse, joystick, touchpad, or trackball, and one would still consider techniques such as double-clicking, dragging, dialogue boxes as being “the same” technique;

  2. The Interchange of devices is not neutral from the perspective of the user:  While the skill of using a GUI with a mouse transfers to using a touchpad, and the user will consider the interface as using the same techniques, nevertheless, the various devices have their own idiomatic strengths and weaknesses.  So, while the user will consider the techniques the “same”, their performance (speed, accuracy, comfort, preference, etc.) will be different from device to device.  Hence, the interactive experience is not the same from device to device, despite using the same techniques.  Consequently, it is the norm for users and researchers alike to swap one device for another to control a particular technique.

Some Attributes

As I stated above, my general rule is that everything is best for something and worst for something else.  The more diverse the population is, the places and contexts where they interact, and the nature of the information that they are passing back in forth in those interactions, the more there is room for technologies tailored to the idiosyncrasies of those tasks.

 

The potential problem with this, is that it can lead to us having to carry around a collection of devices, each with a distinct purpose, and consequently, a distinct style of interaction.  This has the potential of getting out of hand and our becoming overwhelmed by a proliferation of gadgets – gadgets that are on their own are simple and effective, but collectively do little to reduce the complexity of functioning in the world.   Yet, traditionally our better tools have followed this approach.  Just think of the different knives in your kitchen, or screwdrivers in your workshop.  Yes there are a great number of them, but they are the “right ones”, leading to an interesting variation on an old theme, namely, “more is less”, i.e., more (of the right) technology results is less (not more) complexity.  But there are no guarantees here.

What touch screen based “soft machines” offer is the opposite alternative, “less is more”.  Less, but more generally applicable technology results in less overall complexity.  Hence, there is the prospect of the multi-touch soft machine becoming a kind of chameleon that provides a single device that can transform itself into whatever interface that is appropriate for the specific task at hand.  The risk here is a kind of "jack of all trades, master of nothing" compromise. 

 

One path offered by touch-screen driven appliances is this: instead of making a device with different buttons and dials mounted on it, soft machines just draw a picture of the devices, and let you interact with them.  So, ideally, you get far more flexibility out of a single device.  Sometimes, this can be really good.  It can be especially good if, like physical devices, you can touch or operate more than one button, or virtual device at a time.  For an example of where using more than one button or device at a time is important in the physical world, just think of having to type without being able to push the SHIFT key at the same time as the character that you want to appear in upper case.  There are a number of cases where this can be of use in touch interfaces. 

 

Likewise, multi-touch greatly expands the types of gestures that we can use in interaction.  We can go beyond simple pointing, button pushing and dragging that has dominated  our interaction with computers in the past.  The best way that I can relate this to the everyday world is to have you imagine eating Chinese food with only one chopstick, trying to pinch someone with only one fingertip,  or giving someone a hug with – again – the tip of one finger or a mouse.  In terms of pointing devices like mice and joysticks are concerned, we do everything by manipulating just one point around the screen – something that gives us the gestural vocabulary of a fruit fly.  One suspects that we can not only do better, but as users, deserve better.  Multi-touch is one approach to accomplishing this – but by no means the only one, or even the best.  (How can it be, when I keep saying, everything is best for something, but worst for something else).

There is no Free Lunch. 

• Feelings: The adaptability of touch screens in general, and multi-touch screens especially comes at a price. Besides the potential accumulation of complexity in a single device, the main source of the downside stems from the fact that you are interacting with a picture of the ideal device, rather than the ideal device itself. While this may still enable certain skills from the specialized physical device transfer to operating the virtual one, it is simply not the same.  Anyone who has typed on a graphical QWERTY keyboard knows this. 
  
  User interfaces are about look and feel.  The following is a graphic illustration of how this generally should be written when discussing most touch-screen based systems:
  
  Look and Feel
  
  Kind of ironic, given that they are "touch" screens.  So let's look at some of the consequences in our next points.

• If you are blind you are simply out of luck. p.s., we are all blind at times - such as when lights are out, or our eyes are occupied elsewhere – such as on the road). On their own, soft touch screen interfaces are nearly all “eyes on”. You cannot “touch type”, so to speak, while your eyes are occupied elsewhere (one exception is so-called “heads-up” touch entry using single stroke gestures such as Graffiti that are location independent). With an all touch-screen interface you generally cannot start, stop, or pause your MP3 player, for example, by reaching into your pocket/purse/briefcase. Likewise, unless you augment the touch screen with speech recognition for all functions, you risk a serious accident trying to operate it while driving. On the other hand, MP3 players and mobile phones mechanical keys can to a certain degree be operated eyes free – the extreme case being some 12-17 year old kids who can text without looking!

·         Handhelds that rely on touch screens for input virtually all require two hands to operate:  one to hold the device and the other to operate it.  Thus, operating them generally requires both eyes and both hands.

·         Your finger is not transparent:  The smaller the touch screen the more the finger(s) obscure what is being pointed at.  Fingers do not shrink in the same way that chips and displays do.  That is one reason a stylus is sometimes of value:  it is a proxy for the finger that is very skinny, and therefore does not obscure the screen.

·         There is a reason we don’t rely on finger painting:  Even on large surfaces, writing or drawing with the finger is generally not as effective as it is with a brush or stylus.  On small format devices it is virtually useless to try and take notes or make drawings using a finger rather than a stylus.  If one supports good digital ink and an appropriate stylus and design, one can take notes about as fluently as one can with paper.  Note taking/scribble functions are notably absent from virtually all finger-only touch devices.

·         Sunshine:  We have all suffered trying to read the colour LCD display on our MP3 player, mobile phone and digital camera when we are outside in the sun.  At least with these devices, there are mechanical controls for some functions.  For example, even if you can’t see what is on the screen, you can still point the camera in the appropriate direction and push the shutter button.  With interfaces that rely exclusively on touch screens, this is not the case.  Unless the device has an outstanding reflective display,  the device risks being unusable in bright sunlight.

Does this property make touch-devices a bad thing?  No, not at all.  It just means that they are distinct devices with their own set of strengths and weaknesses.  The ability to completely reconfigure the interface on the fly (so-called “soft interfaces”) has been long known, respected and exploited.   But there is no free lunch and no general panacea.  As I have said, everything is best for something and worst for something else.   Understanding and weighing the relative implications on use of such properties is necessary in order to make an informed decision.  The problem is that most people, especially consumers (but including too many designers) do not have enough experience to understand many of these issues.  This is an area where we could all use some additional work.  Hopefully some of what I have written here will help.

 

An Incomplete Roughly Annotated Chronology of Multi-Touch and Related Work

 

In the beginning ....: Typing & N-Key Rollover (IBM and others). While it may seem a long way from multi-touch screens, the story of multi-touch starts with keyboards. Yes they are mechanical devices, "hard" rather than "soft" machines.  But they do involve multi-touch of a sort. First, most obviously, we see sequences, such as the SHIFT, Control, Fn or ALT keys in combination with others.  These are cases where we want multi-touch. Second, there are the cases of unintentional, but inevitable, multiple simultaneous key presses which we want to make proper sense of, the so-called question of n-key rollover (where you push the next key before releasing the previous one).

Photo Credit

 

Electroacoustic Music:  The Early Days of Electronic Touch Sensors (Hugh LeCaine , Don Buchla & Bob Moog). http://www.hughlecaine.com/en/instruments.html The history of touch-sensitive control devices pre-dates the age of the PC A number of early synthesizer and electronic music instrument makers used touch-sensitive capacitance-sensors to control the sound and music being made. These were touch pads, rather than touch screens The tradition of innovating on touch controls for musical purposes continued/continues, and was the original basis for the University of Toronto multitouch surface, as well as the CMU Sensor Frame.

 

1982: Flexible Machine Interface (Nimish Mehta , University of Toronto). ·         The first multi-touch system that I am aware of.  ·         Consisted of a frosted-glass panel whose local optical properties were such that when viewed behind with a camera a black spot whose size depended on finger pressure appeared on an otherwise white background.  This with simple image processing allowed multi touch input picture drawing, etc.  At the time we discussed the notion of a projector for defining the context both for the camera and the human viewer.  ·         Mehta, Nimish (1982), A Flexible Machine Interface, M.A.Sc. Thesis, Department of Electrical Engineering, University of Toronto supervised by Professor K.C. Smith.

 

1983: Soft Machines (Bell Labs, Murray Hill) ·         This is the first paper that I am aware of in the user interface literature that attempts to provide a comprehensive discussion the properties of touch-screen based user interfaces, what they call “soft machines”. ·         While not about multi-touch specifically, this paper outlined many of the attributes that make this class of system attractive for certain contexts and applications. ·         Nakatani, L. H. & Rohrlich, John A. (1983). Soft Machines: A Philosophy of User-Computer Interface Design. Proceedings of the ACM Conference on Human Factors in Computing Systems (CHI’83), 12-15.

 

1983: Video Place / Video Desk (Myron Krueger) ·         A vision based system that tracked the hands and enabled multiple fingers, hands, and people to interact using a rich set of gestures. ·         Implemented in a number of configurations, including table and wall. ·         Didn’t sense touch, per se, so largely relied on dwell time to trigger events intended by the pose. ·         Essentially “wrote the book” in terms of unencumbered (i.e., no gloves, mice, styli, etc.) rich gestural interaction.  Work that was more than a decade ahead of its time and hugely influential, yet not as acknowledged as it should be. ·          Krueger, Myron, W. (1983). Artificial Reality. Reading, MA: Addison-Wesley. ·         Krueger, Myron, W. (1991). Artificial Reality II. Reading, MA: Addison-Wesley. ·         Krueger, Myron, W., Gionfriddo, Thomas., & Hinrichsen, Katrin (1985). VIDEOPLACE - An Artificial Reality, Proceedings of the ACM Conference on Human Factors in Computing Systems (CHI’85), 35 - 40.

Myron’s work had a staggeringly rich repertoire of gestures, muti-finger, multi-hand and multi-person interaction.

 

1984:  Multi-Touch Screen (Bell Labs, Murray Hill NJ) ·         A multi-touch touch screen, not tablet.  It used a transparent capacitive array of touch sensors overlaid on a CRT.  Could manipulate graphical objects with fingers with excellent response time ·         I believe that it was developed by Bob Boie, but was shown to me by Lloyd Nakatani (see above), who invited me to visit Bell Labs after seeing the presentation of our work at SIGCHI in 1985 ·         there was also  one patented by Leonard Kasday of Bell Labs (US Patent 4484179) in 1984, that used optical techniques ·         Since their technology was transparent and faster than ours, when I saw it, my view was that they were ahead of us, so we stopped working on hardware (expecting that we would get access to theirs), and focus on the software and the interaction side, which was our strength. ·         Around 1990 I took a group from Xerox to see it since we were considering using it for a photocopy interface.

 

1985: Multi-Touch Tablet (Input Research Group, University of Toronto): http://www.billbuxton.com/papers.html#anchor1439918 ·         Developed a touch tablet capable of sensing an arbitrary number of simultaneous touch inputs, reporting both location and degree of touch for each. ·         To put things in historical perspective, this work was done in 1984, the same year the first Macintosh computer was introduced.  ·         Used capacitance, rather than optical sensing so was thinner and much simpler than camera-based systems. ·         A Multi-Touch Three Dimensional Touch-Sensitive Tablet (1985).  Videos  at: http://www.billbuxton.com/buxtonIRGVideos.html Issues and techniques in touch-sensitive tablet input.(1985). Videos at: http://www.billbuxton.com/buxtonIRGVideos.html

 

 

1985:  Sensor Frame  (Carnegie Mellon University) This is work done by Paul McAvinney and described in the following paper from 1986r: McAvinney, P. (1986).  The Sensor Frame - A Gesture-Based Device for the Manipulation of Graphic Objects. Carnegie-Mellon University, The device used optical sensors in the corners of the frame to detect fingers. At the time that this was done, miniature cameras were essentially unavailable.  Hence, the device used memory chips with the backs removed for imaging. It could sense up to three fingers at a time (but due to optical technique used, there was potential for misreadings due to shadows. Since the field of view was not just a plane, the device could also could detect the angle that the finger came in to the screen.

 

 

1986:  Bi-Manual Input  (University of Toronto) In 1985 we did a study, published the following year,  which examined the benefits of two different compound bi-manual tasks that involved continuous control with each hand The first was a positioning/scaling task.  That is, one had to move a shape to a particular location on the screen with one hand, while adjusting its size to match a particular target with the other. The second was a selection/navigation task.  That is, one had to navigate to a particular location in a document that was currently off-screen, with one hand, then select it with the other. Since bi-manual continuous control was still not easy to do (the ADB had not yet been released - see below), we emulated the Macintosh with another computer, a PERQ. The results demonstrated that such continuous bi-manual control was both easy for users, and resulted in significant improvements in performance and learning. See Buxton, W. & Myers, B. (1986).  A study in two-handed input. Proceedings of CHI '86, 321-326.[video] Despite this capability being technologically and economically viable since 1986 (with the advent of the ADB - see below - and later USB), there are still no mainstream systems that take advantage of this basic capability.  Too bad. This is an example of techniques developed for multi-device and multi-hand that can easily transfer to multi-touch devices.

 

 

1986:  Apple Desktop Bus (ADB) and the Trackball Scroller Init (Apple Computer / University of Toronto) The Macintosh II and Macintosh SE were released with the Apple Desktop Bus.  This can be thought of as an early version of the USB. It supported plug-and-play, and also enabled multiple input devices (keyboards, trackballs, joysticks, mice, etc.) to be plugged into the same computer simultaneously. The only downside was that if you plugged in two pointing devices, by default,  the software did not distinguish them.  They both did the same thing, and if a mouse and a trackball were operate at the same time (which they could be) a kind of tug-of-war resulted for the tracking symbol on the screen. My group at the University of Toronto wanted to take advantage of this multi-device capability and contacted friends at Apple's Advanced Technology Group for help. Due to the efforts of Gina Venolia and Michael Chen , they produced a simple "init" that could be dropped into the systems folder called the trackballscroller init.  It enabled the mouse, for example, to be designated the pointing device, and a trackball, for example, to control scrolling independently in X and Y.  See, for example, Buxton, W. (1990). The Natural Language of Interaction: A Perspective on Non-Verbal Dialogues.In Laurel, B. (Ed.). The Art of Human-Computer Interface Design, Reading, MA: Addison-Wesley. 405-416. They also provided another init that enabled us to grab the signals from the second device and use it to control a range of other functions. See fr example, Kabbash, P., Buxton, W.& Sellen, A. (1994). Two-Handed Input in a Compound Task. Proceedings of CHI '94, 417-423. In short, with this technology, we were able to deliver the benefits demonstrated by Buxton & Myers (see above) on standard hardware, without changes to the operating system, and largely, with out changes even to the applications. This is the closest that we came, without actually getting there, of supporting multi-point input - such as all of the two-point stretching, etc. that is getting so much attention now, 20 years later.  It was technologically and economically viable then. To our disappointment, Apple never took advantage of this - one of their most interesting - innovations.