P is for pen-based computing. It is easy to forget that 20 years ago, few people used a qwerty keyboard. Apart from hacks and writers, most who did were women, and in low-status jobs; executives used flashy fountain pens, or gave dictation. When computers began to appear, it was difficult to get executives to use them.

One proposed solution was computers that you could write on: if the machine could recognise handwriting, even high-powered people would use them. Making pen-based screens is simple, using a touch-sensitive membrane that detects pressure from a stylus, or an electrified surface that detects a voltage difference between the stylus's tip and points on the screen.

However, people found it easier to learn to type than they had expected; and computers, like people, have trouble recognising the vagaries of different handwriting. It takes a lot of processing power.

Thus when Apple launched the Newton in autumn 1993, with a pen and a handwriting recognition system, everyone thought it had a world-beater - until they experienced its amazingly inaccurate translation of whatever you might write on it.

Yet pen-based computers have real uses. They are terrific for people such as engineers who have to fill in long forms on the move: in such situations, keyboards are more of a problem than a help.

But keyboards hold sway, as shown by their presence on the modern consumer toy, the personal digital assistant. Machines such as the Psion 3a are barely big enough to type on - yet people do so happily. The Newton struggles on, but has never been a serious contender. It doesn't, after all, have a keyboard.