The science of keyboard design

The Handbook of Human–Computer Interaction, edited by Martin Helander, Thomas Landauer and Prasad Prabhu, is a book published in 1997 that attempts to summarise research relevant to the design of interactive software and hardware. Its 62 chapters fill 1500 pages and provide advice on a multitude of topics, covering analysis, design and evaluation of interactive systems, as well as the psychological and ergonomic underpinnings of human–computer interaction. One of those chapters is titled Keys and Keyboards and was written by James Lewis, Kathleen Potosnak and Regis Magyar. It considers virtually every imaginable factor involved in designing keyboards, and, by drawing from experimental studies, provides recommendations for each of them. Ever since I read this chapter a couple of years ago, I've been meaning to summarise some of their conclusions and to consider them in the context of modern keyboard design. The recent string of keyboard-related links on Daring Fireball (e.g. here, here, here, here, here and here) moved me to finally sit down and do it, so here it is.

Clickiness

One of the most-cited criteria for keyboard aficionados to prefer a certain keyboard over another is “clickiness”. The idea behind this is that a good keyboard should give you some tactile feedback when you've successfully “actuated” a key, and that you shouldn't have to depress the key all the way to the bottom to be sure, as this would not allow you to type very fast. Some keyboards don't click at all, some give a softer and others a sharper click. The exact behaviour can be described by a graph plotting how the physical force required to push the key varies along its way down and its way up. The sudden dip in force on the downstroke is where you will feel the “click”.

I have to agree with many others that the Apple Extended Keyboard II is the finest keyboard I have used in terms of key feel. I used it for a couple of years in the late 90s, and it was the first to even make me aware of how good a keyboard could feel. I have not found a match since. In 2004 I bought a Matias TactilePro, which supposedly uses the exact same mechanical switches as the Apple Extended Keyboard II. However, it seemed to require more force, wasn't as smooth and seemed more noisy. Keys also kept failing, so I eventually gave up on it and now use a Macally icekey, which uses scissor-switch keys like those found on a laptop, but feels a bit firmer. I have also been quite impressed with a standard HP keyboard that I use at work. It uses rubber dome switches, which usually have a much inferior feel to mechanical or scissor switches, but they are square rather than round, which seems to give it an acute but soft clickiness and a very pleasant overall feel.

Hysteresis

I already mentioned that the key actuates when the click happens, but when should it deactuate? The obvious choice would be to make it the same as the actuation point:

However, this behaviour leads to an unexpected problem: any tiny amount of vibration around the actuation point can mean that the switch briefly fluctuates between the open and closed states, causing, for example, a letter to be inserted twice on one key press. To avoid this happening, keys should have to return some way (0.25–1.5 mm) before deactuating. This distance is called hysteresis:

Astonishingly, plenty of modern keyboards seem to have no hysteresis built into their switches. I get duplicated letters more than occasionally when typing on my Macally icekey or on my G4 iBook. The MacBooks don't seem to use hysteresis either. I haven't had a chance to test one of the new aluminium Apple keyboards or a MacBook Pro for this.

Travel and force

Another common factor mentioned when assessing keyboards is the distance the keys travel. Laptop keyboards and laptop-like keyboards tend to have shorter travel than most desktop keyboards. Related to travel is the amount of force required to depress a key. The Handbook of HCI recommends key travel to be between 1.3 and 6.4 mm, and the key force to be between 28 and 142 g. These are rather wide ranges, and in fact experiments showed that users are not too fussed about these two variables and will simply adapt their typing behaviour accordingly.

Keyboard profile

The book chapter summarises the results of two studies looking into keyboard profiles. The dished profile seemed to narrowly come out on top, with higher throughput than the stepped profile in one study. The flat profile performed worst (ahem), with more errors going undetected by typists and with lower throughput.

Key shape

The vast majority of keyboards have key caps that are either concave or dished. Apple however, has started giving their keyboards flat keys. Unfortunately, the book doesn't give any recommendations in this regard. However, I have my own little theory about this. I think that key shape matters because the direction at which you strike a key is rarely perfectly parallel with the key's travel path. Most of the force you apply may go into moving the key down, but the rest will go into pushing the key sideways and into pushing your finger along the surface. The less force goes into moving your finger sideways, the more force goes into the key. A concave key cap directs more of the force into the key when you strike it an angle, which effectively increases the area that you can comfortably use to depress the key.

As I said, this is purely a theory, and I have no evidence that the key cap shape actually makes any difference. I haven't spent enough time on one of the newer Apple keyboards to really form an opinion about them.

Other factors

Some of the other factors to consider in keyboard design are tilt, key size and key spacing.

All users in the studies referred to preferred having some tilt rather than a completely horizontal keyboard. 50% preferred an angle between 15˚ and 25˚. Not surprisingly, the preferred angle correlates with the seat height and with the user's stature.

Keys should be no less than 19 mm apart, as more tightly spaced keys tend to slow users down. There were no conclusive results on key size, but the ANSI standard recommends a size of 12 mm × 12 mm.

Final thoughts

Clearly, a fair amount of work, time and research funding went into this topic, especially during the years when computers were less of a commodity and still more of a research topic. The work successfully produced empirical evidence that led to very concrete and specific recommendations for the design of keyboards. Even so, today's keyboard makers seem either oblivious or ignorant of all this latent wisdom. Given that we're talking about a device that millions of people have to physically interact with on a daily basis – in many cases for several solid hours – it is sad that we've kept repeating the same mistakes that we've had the potential to avoid for more than twenty years.