Roughly ...<p>They were going to use a length such that a pendulum would have a 1 second half period, or 2 second full period. That varies according to where you are, but it's pretty good. This means that <i>g=(pi)^2,</i> because the period of a pendulum T is given by <i>2(pi)sqrt(L/g)</i>.<p>Since that varies according to the local value of <i>g</i>, they then used one ten-millionth ( 10^-7 ) of the distance from the North Pole to the Equator running through Paris. They actually did that by extrapolating a shorter distance, but it means the Earth is, "by definition", 40 million metres circumference. They created a platinum bar of the right length and declared that to be the answer.<p>Turns out it's wrong, but they never bothered to fix it.<p>However, in 1960 the eleventh CGPM defined the metre to be 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum. Obviously much easier to use.