A notation used to identify the wheel arrangement of locomotives by specifying first the number of leading wheels, then the number of coupled/driving wheels, and finally the number of trailing wheels. For example 0-6-0 means no leading wheels, six coupled/driving wheels, and no trailing wheels, 4-6-6-4 indicates four leading wheels, two sets each of six coupled/driving wheels, and four trailing wheels. This notation is commonly used for steam locomotives.