Southpaw is a boxing term that designates the stance where the boxer has his right hand and right foot forward, leading with right jabs, and following with a left cross right hook. Southpaw is the normal stance for a left-handed boxer.
Because the American system of measurement is very similar to the British imperial system, units are also commonly referred to as imperial units. The units have many different origins.
So I guess to summarize SAE mechanical tools are those that are measured in a fraction of an inch and metric are those mechanical tools measured in millimeters. SAE stands for: Standard American Equivalent.