Although forecasts of wind-speed are usually couched in miles per hour or "forces" of the Beaufort Scale, the average meteorologist thinks of the wind in terms of knots, or if he or she is engaging in research, metres-per- second will be the units used. These are just four examples of the complexity of answering age-old questions of the kind "How long is a piece of string?". It depends on the units of measurement you care to use.
Magna Carta, signed reluctantly by bad King John in 1215, contained in Latin a clause which required that "throughout the Kingdom there shall be standard measures of ale, wine and corn, and weights are to be similarly standardised."
But the barons were less successful when it came to fixing the value of the coinage of the realm until the "Easterlings" arrived.
The Easterlings were merchants from north Germany who settled in London a generation after Magna Carta, and their money then, as now, was considered to be particularly sound. Two hundred and forty of their silver pennies, or "easterlings", became known as the pound sterling, because that, in Troy measure, was precisely what they weighed.
This tradition of 240 pennies to the pound continued until comparatively recently. There was an overture for decimalisation in 1798, when the ubiquitous Tallyrand invited the London government to send a deputation to Paris to "deduce an invariable standard for all the measures and for all the weights".
But naturally enough his impertinence was totally ignored. It was not until 1971 that the decimal system was finally adopted on these islands.
As regards capacity, the old English system was based - somewhat unhygienically - on the mouthful. Two mouthfuls made a jigger, two jiggers a handful or an ounce, and two handfuls a jack or jackpot. Successive doublings brought the jill, the cup, the pint, the quart, the pottle and the gallon, and so on, through the bushel and the coomb up to a tun.
Then Charles I imposed a tax on goods sold by the jack, and later reduced the size of this measure so that the tax would produce more revenue, thus reducing the size of the jill as well. The angry populace registered their protest at this action in the form of what is now a well-known rhyme, and which has nothing to do, it seems, with the two accident-prone youngsters of the narrative:
Jack and Jill went up the hill
To fetch a pail of water.
Jack fell down and broke his crown,
And Jill came tumbling after.