The circumference of some unit wheel. If you make a wheel that is 1 unit in diameter in any unit that you please and measure things by placing a mark on the wheel where it touches ground at your start point and counting off full revolutions as you roll the wheel, stopping when that same mark is at the bottom then your measurements are always going to be count X size of unit X pi. If you do more than one such measurement in total then all of your measurements are going to be divisible by the size of unit and pi. If there’s any record of what that unit is (and if you engage in a lot of trade there just might be), then the whole thing becomes kind of obvious when someone converts to the lengths to your units and always gets a multiple of pi.