The most common system used to describe a steam locomotive's wheel arrangement is the Whyte system, which consists of three digits: the first represents the number of leading carrying wheels, the second denotes the number of driving wheels and the last refers to the number of trailing carrying wheels. The number may also be followed by a letter as follows: T = side tank engine, ST = saddle tank, PT = pannier tank and WT = well tank. Some European railways used the Whyte system, except that the number of axles is used instead of the number of wheels: 442 would therefore become 221. Letters were also introduced to denote driving axles, with numbers then referring to carrying axles (for example, CC = two groups of 3 driving axles, and 2D2 = two lots of two carrying axles with four driving axles in the middle).