The USA uses what are termed(USCU), which derive from the British Imperial system as described in this Wikipedia article...A quote from the above article...Tools and fasteners with sizes measured in inches are sometimes called "SAE bolts" or "SAE wrenches" to differentiate them from their metric counterparts. The Society of Automotive Engineers (SAE) originally developed fasteners standards using U.S. units for the U.S. auto industry; the organization now uses metric units.touches on the use of "SAE", which has become something of an unofficial, erroneous synonym for the more correctUSCU is ponderous enough to argue strongly for a one word descriptor. I prefer my own portmanteau, inferial, a combination of infuriating and Imperial.