The unit described on this page was in use prior to modern methods of precision measurement. It was based on a standard which is no longer available, and which is not capable of being calibrated against modern measurement units. Therefore, the equivalence to modern SI units or to current United States customary units can only be considered approximate.
The perche (sometimes rendered perch in English) was a subsidiary unit of length in the French pre-revolutionary system, defined as 3 toises. Its value varied from place to place within France, but the values used in Paris are generally taken as the standards in reading older literature. (Note that the termperche was also used for a unit ofarea.)