Under Bulleid, a new continental system of numbering was introduced for his own locomotives, based upon his experiences at the French branch of Westinghouse Electric before the First World War, and his tenure in the rail operating department during that conflict. The Southern Railway number adapted a modified UIC classification system where "2" and "1" refer to the number of un-powered leading and trailing axles respectively, and "C" refers to three driving axles (the system was only applied to new 6-coupled locos and one Co-Co electric loco before nationalisation). As an example, the first Merchant Navy class locomotive was numbered 21C1.