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Measurement systems (once upon a time) were arbitrary and local - a foot, say, corresponded to the length of the foot of the king. This led to a serious anomaly after the French revolution, when there was no longer a king whose foot could be measured.

Instead, a new system of measurement was introduced - the metric system. A metre was originally defined as one 10,000,000th part of the distance between the north pole and the equator (on a meridian through Paris). It has since been refined to have a less Paris-centric definition based on atomic wavelengths.

In any case, a yard was eventually standardised to be 0.9144m, so that a metre is 1.0936 yards or 3.2808 feet.

People soon realised that multiplying things by ten each time was far easier than remembering that there were 12 inches in a foot, of which there were three in a yard - not to mention that 220 yards made a furlong and eight of those was a mile. You didn't even need your times tables, you just shifted the decimal place. They soon extended the concept to other quantities you could measure - in mass, for instance, a kilogram is approximately 2.2 pounds (so that 0.454kg is a pound) - and is the mass of a litre of water. This is the subject of probably my favourite maths joke: a friend of a friend was confronted with a question involving a thousand litres of water. "Crikey," he said, "that must weigh a ton!"

So what's a litre? In metric terms, it's 1000 cubic centimetres, or the volume of a cube with 10cm on each side. A (British) pint is 0.578l, making each litre 1.73 pints. A gallon (eight British pints) is 4.54l, although that's not the case in America. Even though the imperial system is known stateside as "British units", the American gallon is a little smaller, being only 3.8l or so.

In Britain, the mile (a little over 1.6km) and the pint are the only imperial units that remain in regular use. Ireland (being Ireland) has its speed limits in miles per hour but has signs giving distances in kilometres.

Reputedly, Burma is the only other country in the world where the imperial system is not seen as a quaint relic of more interesting mathematical days. In defence of imperial units, language would be much poorer without phrases such as "give them an inch...", "a quart in a pint-pot" and "a pound of flesh". Walking a kilometre in someone else's shoes would be far less of an ordeal.

Learn more about this author, Colin Beveridge.
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