Under the Whyte notation for the classification of steam locomotives, represents the wheel arrangement of an articulated locomotive with two separate swivelling engine units, arranged back to back with the boiler and cab suspended between them. Each engine unit has two leading wheels in a leading truck, six powered and coupled driving wheels on three axles and no trailing wheels. A similar wheel arrangement exists for Mallet steam locomotives on which only the front engine unit swivels, but these are referred to as 2-6-6-2.