summerize this The metre, specifically, was originally defined as 1/10,000,000 of the distance from the equator to the North Pole. (This is why the circumference of the Earth is a nice, round number: 40,000km.) This was in Revolutionary France, in 1793, when they got fed up with having to deal with feet and inches that in extreme cases got re-defined by referring to the actual body of the current ruler. They wanted a system that would be as independent and neutral as possible, and that would ideally also work internationally. In reality, it actually didn't turn out that neatly: the measurements of the distances weren't accurate enough, and it was a bit cumbersome to deal with in practice. They quickly re-defined the metre to be the length of a particular prototype stick that they had in Paris. In 1960, that definition was also found to be imprecise (and the prototype sticks around the world occasionally had to be re-calibrated), and the metre was re-defined to be a certain number of waves of light at a certain frequency (think "colour"). This was accurate enough for very precise scientific measurements. Only 23 years later, they ditched that one, too, and re-defined it