Found on this site:
http://www.sheldonbrown.com/gloss_w.html
Sir Joseph Whitworth was one of the great inventors of the 19th century. He invented the milling machine, and was the first to manufacture all-metal machine tools. I believe that he was also connected with the Rudge bicycle company (later acquired by Raleigh).
He was also one of the first to establish a standardized set of screw threads and wrench sizes. Whitworth fasteners were used in British industry up until the late 1960's, when Britain adopted the metric system. The Whitworth system is now virtually extinct.
Whitworth fasteners used a 55 degree thread angle. The wrench sizes were confusingly marked, the wrenches are all larger than you would think. This is because the dimension given on a Whitworth wrench is the diameter of the bolt thread usual for that size wrench, rather than the size of the head. Some Whitworth wrenches even have two marked sizes, because the same head size is used with one coarse thread and a different diameter fine thread.
Some of the smaller sized Whitworth threads are interchangeable with S.A.E. threads.