======================================= the right number of buttons for a mouse ======================================= by Andrew Main (Zefram) 2005-05-15 abstract -------- Manufacturers of computer input hardware appear to be confused about how many buttons a mouse should have. A simple ergonomic analysis shows that the correct answer is three. 0. table of contents ==================== 0. table of contents 1. problem 2. solution 2.0. principles 2.1. basic design 2.2. additional ergonomic considerations 3. references 1. problem ========== Mice are currently available with a wide variety of numbers and types of buttons. The number of buttons may be one (Apple's infamous design), two (older PC mice), three (Unix mice and newer PC mice), four [THINKING] [ULTRAX], five [ELITE] [FELLOWES-5], six [VERSATILE], seven [DIAMOND], eight [MX700], nine [CLAW], ten [STEALTH-3D], eleven [EXPERT], or probably more. These buttons may be on the top, back, or either side of the mouse. Increasingly commonly, one of the buttons is not just a simple button but a vertically oriented wheel. In addition to being pressed as a button, this "scroll wheel" may be rotated forward or backward in quantised increments. A recent variation of this can furthermore be tilted left or right [TILT-WHEEL]. Most current mice are awful to use. The usual sleek exterior shape means that the digits are not only deprived of any tactile clue as to where they should be but are also liable to slide off the domed surface. Mice often have a very smooth plastic surface, exacerbating the tendency for digits to slip. Some manufacturers even hide the buttons under a slightly flexible exterior, so that there is no visible gap between the buttons and the rest of the surface, thus eliminating all tactile clues as to the buttons' location. Mice with exactly two buttons require an awkward finger posture: either the index finger and the middle finger are spread wide apart, with the ring finger and little finger crowded together, or the middle finger rests between the buttons, overlapping both and interfering with button presses. On mice with many buttons, digits assigned more than one button must move around independently to operate the buttons, tending to move the carpus and thus move the mouse as a whole. If a digit pressing a button is also one of those gripping the mouse then this tendency is worse. Scroll wheels are nastier still. The wheel projects from the surface of the mouse, so a finger resting on it is more extended than the fingers next to it, risking static loading injury. Being a domed bump in the surface, and usually narrower than a finger, the wheel is the most awkward possible shape to rest a finger on: the finger cannot be relaxed lest it slip off the wheel, but must instead be actively held in the least comfortable place possible, at the apex of the bump. The user must then be careful not to press the wheel by accident. A wheel is also optimised to make one more likely to press an adjacent button in addition to the wheel. [PDS-THREE] discusses the issue that scroll wheels make very poor buttons. As for using the wheel as a wheel: first consider that while rotating the wheel the user must be careful not to accidentally press the wheel as a button, and vice versa, for these two modes of use interfere with each other. The motion required to rotate the wheel is an unnatural combination of coordinated flexor and extensor usage on a single isolated digit. Usually it is the middle finger that is called upon to engage in this isolation exercise, to which it does not take well, especially when so unusually extended relative to the adjacent fingers. The scroll wheel is, in summary, optimised to cause repetitive strain injury. 2. solution =========== 2.0. principles --------------- The problems of current mice are caused by the application of bad design principles. The included input devices are often chosen with specific functions in mind (the "scroll wheel" is a prime example), even though whatever form the device takes it will find multiple uses in different contexts. Often, many aspects of the shape of a mouse are dictated by aesthetics, even though the shape has great functional effects. The proper design principle to adhere to is to design around the hand's capabilities. There should be exactly as much input hardware as the hand can conveniently operate. Once the mouse interface has been determined by these considerations, what use to make of that interface may be considered separately. The shape of the mouse should be dictated by functionality first, with aesthetics relegated to aspects of the design that have no functional impact. The remainder of this section will apply these principles to mouse design. Only one-handed mice for anatomically normal human hands will be considered; variations for abnormal anatomy are beyond the scope of this essay. 2.1. basic design ----------------- The ergonomics of mice is dominated by the need for the user to hold the mouse in order to move it around, which is the primary form of mouse input. All mouse buttons and other input devices on a mouse must be operated by a hand which is principally occupied in holding the mouse, so there is not the freedom of movement that a hand on a keyboard, for example, enjoys. The hold on the mouse must be stable enough for precise pointing, so the range of usable input actions is limited not only by what the digits can physically achieve but also by the need to avoid affecting the grip on the mouse. For a stable hold and control of movement, the mouse must be gripped between (at least) two digits. The thumb, being the reversible digit, must be one of the gripping digits, on the medial side. On the lateral side the best possible control is achieved by using the digit closest to the base of the mouse, namely the little finger. This analysis applies regardless of whether the metacarpus is to be oriented horizontally (as is conventional) or vertically (as in some ergonomic mouse designs). To press a button with one of the digits that is gripping the mouse would inevitably compromise the hold on the mouse. Any such button press leads to slight movement of the mouse during the pressing action, much as pulling the trigger on a handgun changes the aim slightly. A good mouse therefore should not assign any buttons or other input devices whatsoever to the thumb or little finger. This leaves three digits available to press buttons. Their freedom of movement is curtailed by the general hand position. Essentially, they are constrained to lie on the top (or, if the metacarpus is not horizontal, on the lateral side) of the mouse. In this position no complex movement is comfortable, and the only reasonable input action is a slight flexion of the digit to depress a button with the distal phalanx. This flexion may be accomplished using either the deep flexor or the superficial flexor, or a combination of the two. Thus the ideal mouse has exactly three buttons, one operated by each of the index finger, the middle finger, and the ring finger. The highly unnatural movement required by a scroll wheel rule out any of these buttons being a scroll wheel. Thus each of the three buttons is a plain single-action button rather than anything more complex. 2.2. additional ergonomic considerations ---------------------------------------- Ideally, in the quiescent state (no buttons being pressed) the button fingers should be completely relaxed. Thus the mouse surface should be slightly curved from front to back, allowing the fingers a relaxed slightly curved state. All the buttons should be approximately coplanar, avoiding any button finger being significantly more extended than any other. The entire surface of the mouse that is in contact with the hand should be smooth, with no bumps such as scroll wheels provide. There should be shallow grooves on the buttons, similar to the grooves on keyboard keytops, to guide the fingers into place and to help them maintain correct position while relaxed. Some mice that meet these shape constraints are illustrated in [PMO]. The buttons must require sufficient actuation force that the weight of the relaxed fingers does not press them. Once pressed, it should be possible to keep a button pressed with a minimum of force. However, the return force should be sufficient that merely relaxing the flexors should achieve button release, without requiring any use of the extensor. It is possible for a good ergonomic mouse design to be bilaterally symmetric, making it equally usable by either hand. There is considerable advantage in doing so. Where further refinement of the ergonomics dictates a chiral design, manufacturers should bear in mind that not all mouse users use the same hand, so both chiralities should be made available. Some speculative interface designs involve using two mice simultaneously, one in each hand, thus requiring both chiralities at once. 3. references ============= [CLAW] SLCentral, "Ferraro Designs Claw Review", March 13th, 2001, . [DIAMOND] XYZ Computing, "Razer Diamondback Optical Mouse", Thursday, 23 December 2004, . [ELITE] CompUSA.com, "Kensington Optical Elite Five-Button Mouse" (advertisement), . [EXPERT] Kinesis Corp., "Kensington Expert Pro" (advertisement), . [FELLOWES-5] CleanSweepSupply.com, "Five-Button Optical Mouse with Microban Protection by FELLOWES" (advertisement), . [MX700] HardwareCentral, "Logitech MX700 Cordless Optical Mouse Review", Thu 10/10/02, . [PDS-THREE] Peter da Silva, "Where are all the three button mice?", FEB 2003, . [PMO] Contour Design, Inc., "Contour Perfit Mouse Optical Home Page" (advertisement), . [STEALTH-3D] ABC Software Developers, "Stealth 3D Mouse Developer Guide", . [THINKING] Nova International, "Thinking Mouse" (advertisement), . [TILT-WHEEL] Microsoft Corporation, "Microsoft Mouse and Keyboard Hardware - Tilt Wheel Technology", . [ULTRAX] EXcaliberPC.com, "Logitech UltraX Mouse optical 4 button(s) wired PS / 2, USB" (advertisement), . [VERSATILE] Directron.com, Inc., "VersatileScroll Mouse" (advertisement), .