In 1922 the Deutsche Reichsbahn began to develop a renumbering plan to standardize the numbering of steam locomotives that had been taken over from the state railways (Länderbahnen). Its basis was the corresponding DRG classification system.
The first renumbering plan in 1922 envisaged more class numbers than the later plans. The development of this scheme was discontinued because it was seen that there would be problems in practically adopting it.
The second, provisional, renumbering plan of 25 July 1923 was very like the final version of 1925 in its basic structure. It incorporated space for the new standard locomotives (Einheitslokomotiven) that were planned. The third and final renumbering plan of 1925 differed from its predecessor primarily in that all the locomotives retired up to that point – in some cases entire classes – were deleted; in addition several mistakes in the numbering were corrected.
With the exception of Bavarian classes, new locomotives built after 1923 were delivered from the outset with their new operating numbers. The actual renumbering of older engines did not begin until 1926.
Locomotives that were seen as obsolete and which would therefore soon be retired, were given serial numbers from 7001 after their class number.