Whyte notation
The Whyte notation for classifying steam locomotives by wheel arrangement was devised by Frederick Methvan Whyte, and came into use in the early twentieth century following a December 1900 editorial in "American Engineer and Railroad Journal". The notation counts the number of leading wheels, then the number of driving wheels, and finally the number of trailing wheels, groups of numbers being separated by dashes. Other classification schemes, like UIC classification and the French, Turkish and Swiss systems for steam locomotives, count axles rather than wheels.
UIC classification of locomotive axle arrangements
The most common wheel arrangements in modern locomotives are Bo′Bo′ and Co′Co′.Standard practice in the United Kingdom was to use the Whyte notation for steam locomotives. During World War 2, the Southern Railway used a system modified from the UIC method: Oliver Bulleid, the Chief Mechanical Engineer, numbered the first nineteen of his 4-6-2 "Merchant Navy" class Pacifics 21C1 to 21C19, and the first seventy of his 4-6-2 "West Country and Battle of Britain" class Light Pacifics 21C101 to 21C170, referring to leading axles, trailing axles and powered axles (cf. UIC classification 2′C1′, Whyte 4-6-2). Also, all forty of Bullied's "Austerity" 0-6-0 "Q1" class were similarly numbered C1 to C40, and his two electric locomotives (later British Rail Class 70) were numbered CC1 and CC2 (UIC classification Co′Co′, Commonwealth Co-Co).