Whereas the Whyte notation counts wheels, the UIC notation counts axles. Driving and non-driving wheels are distiguished the use of upper case letters of the alphabet (starting at A for a single axle) to denote driving wheels. For example a Pacific-class steam locomotive, denoted 4-6-2 in the Whyte notation, would have a UIC designation of 2C1. On most locomotives (except very early steam locomotives) non-driving wheels are mounted on steerable bogies - this is marked using apostrophes in UIC notation.
Articulated locomotives are indicated using brackets. Mallet locomotives are indicated by bracketing the front power unit - for example, the Union Pacific Big Boy - denoted 4-8-8-4 in Whyte notation, is designated (2'D)D2' in UIC notation. Garrett-type locomotives are indicated by bracketing all individual units.
On many electric and diesel-electric locomotives, axles are individual driven by electric motors - this is indicated by suffixing the driving wheel letter with a lower-case 'o'.
The designation can also have additional suffixes: