When SNCF was created in 1938, the state took over locos from old companies with five different number schemes. The French network consisted of five "regions", numbered from 1 to 5 : 1 = Est, 2 = Nord, 3 = Ouest, 4 = Sud-Ouest, 5 = Sud-Est. Direction of steam division was given to ex-PLM engineers, so they keep the PLM numbering scheme. The first digit is for wheel arrangement (140, 231, etc...). The letter following is for the batch (231 D is the fourth batch of pacific), and after that is the individual number of the loco. Each region gets its own numbering scheme using this system. So a 231 C of region 2 is different from a 231 C of region 4 or 3.
The only letters used in community for both regions are P and Q (unified locos built on SNCF order), R and U (american locos delivered after WW II), and X, Y and Z (german locos captured after WW II).

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