In the field of railroad locomotives, how the wheels of the locomotive are arranged by type, position and connections. There are several notations used to describe wheel arrangements. In the United States and Great Britain, the Whyte Notation is generally used for steam locomotives. In Europe, the UIC Classification scheme is generally used, especially for diesel and electric locomotives. British practice uses a slightly simplified form of that notation for diesels and electrics; the US uses an even more simplified version for such purposes.
Especially in steam days, wheel arrangement was an important attribute of a locomotive, because there were many different ones, each optimised for a different use. Modern diesel locomotives are much more uniform.