Telling the time in two batches of 12, from midnight to midday and midday to midnight, seems so natural to us that we rarely if ever think about why we do so. But a few moments' reflection should tell us that it's actually a highly illogical thing to do and, unless we take steps to compensate for its illogicality, can lead to enormous confusion.

If you want to be sure of catching the right train, bus or aeroplane, it's useful to know whether it departs at, say, six o'clock in the morning or six o'clock in the evening. To do this it is necessary to append an 'a.m.' or a 'p.m.' to the time, or - more logically - to use the 24 hour clock. Six in the morning remains six o'clock, but six in the evening becomes the less friendly 1800 hours.

The real question is why we don't just use the 24 hour clock all the time. No-one really knows, but a number of reasons have been put forward. For a start, in the middle ages, when the first clocks began to appear on churches and public buildings, they were powered by an elaborate system of weights. If a 24 hour clock had been used more power would have been required to sound the hours between 12 and 24. This would have meant more frequent winding, and since winding one of these clocks is hard labour conserving power was a priority.

Another possible reason is that people would have been unable to keep count of the number of strokes if clocks had sounded more than 12 hours. And it is arguably more difficult to read a clock face that has 24 hour markings on it than one that has only 12. Meanwhile, on a 24 hour clock face, an hour hand that moves erratically has less margin for error; with the face divided into 24 segments, there is half the amount of space between hours. So if an hour hand moved too far out of alignment it could look as if it was a full hour ahead of or behind the correct time.

In fact early clocks made in Italy did tend to use a 24 hour system. But, it is argued, the greater precision and technical accomplishment of clocks from northern Europe, where the 12 hour clock was favoured, meant that the latter system of time keeping gradually spread throughout Europe and the rest of the world.

Among the other reasons for adhering to a day that consists of two groups of 12 hours is the way in which time was measured before the advent of modern precision time keeping. The easiest way to determine the time was to observe when the sun reached its highest point. This would give a reference point from which the time until midnight could be measured. It made sense in this context to start counting the hours from one to twelve. Once it became common to measure the length of the day from one midnight to the next, however, it would have seemed illogical either to call the first hour of the day 13, or to start start counting the hours from midday to midnight as 13 to 24. The not very satisfactory solution was to count from midday to midnight and then start again counting from midnight to midday.

Once this system became established it became so widely accepted as the standard way of telling the time that it has proved almost impossible ever since to persuade people to adopt the 24 hour clock in their everyday lives. So it remains a time system restricted to use mainly by the military and the makers of public transport timetables.