A metre, as everybody knows, is just a little over three feet long. Or, if you want to be more precise, you can say it is 1,650,763.73 wavelengths in vacuo of the orange-red line of the spectrum of krypton-86.
But for many years it was thought of as something in between, as the distance between two marks on a platinum-iridium bar stored in the Observatory of the International Bureau of Weights and Measures at St Cloud, near Paris. And this in turn was reckoned to be one 10 millionth part of the distance from the North Pole to the Equator along the meridian of longitude through Paris.
Until the French Revolution, units of length - or indeed of anything else - were more or less whatever the local ruling monarch thought they ought to be. In 1592, for example, it was allegedly decreed that the English yard was to be the length of Queen Elizabeth's "ruling arm". Similarly, every European region developed its own individual idea of a "foot": the shortest was the Hessischer Fuss at only 25 cm, while the longest in Europe was the Paris foot at 32.48 cm.
Even when adequately and accurately defined, units like these are very difficult to use. In the traditional system of these islands, for example, the unit of length is the foot and the unit of weight is the pound; the number of feet in a yard, a furlong or a mile bear no common relation either to each other or to the number of pounds in a stone, a hundredweight or a ton.
All this makes calculations and comparisons hard to carry out, as was obvious in France at the time of the fall of the Bastille. Early in the Revolution, in 1790, a committee of five distinguished persons was established to find a more amenable solution. Overtures were even made across the channel, when the ubiquitous Talleyrand invited the London government to send a deputation to Paris to "deduce an invariable standard for all the measures and for all the weights"; naturally enough, his impertinence was totally ignored.
So the French authorities went on alone, and the result was the metric system that we know today. The various hierarchies of weights and measures are logically connected, and the scale of numeration is the same for nearly all the units, involving merely moving the decimal to the left or right.
The newly devised "metre" and its associated measures were adopted as the official units of the new Republic, and exactly 200 years ago today, on June 22nd, 1799, the new metric standards were ceremoniously deposited in the French archives.