Melbourne's tram classification system is based on classes originally devised by the Melbourne & Metropolitan Tramways Board (M&MTB).^[1] At first this was largely based on the order in which the original tramway operators had introduced each different type of tramcar between 1906 and 1920.^[1]

The classification system was simplified in 1928^[1] (or possibly earlier). All the drop-end combination trams of the B, H, J, M, and S classes were added to A class. The straight sill combination classes F, K, Q, and R, were added to G class. Drop-end-and-centre Maximum traction bogie combination classes D, E, N, and P, were added to C class. Sometimes variations on a design were given extra letters or numerals. For example, W type trams classified as W3, CW5, or SW6. Over the years many of the trams were modified and then reclassified – for example, every one of the original 200 W class trams were converted to W2 class between 1928 and 1933.^[1]

Individual tram fleets had been numbered (from 1 upwards) by each of the pre-M&MTB tramway operators, but by 1924 the M&MTB had consolidated the numbering across the combined fleet.^[1] With the introduction of Z class trams, the M&MTB started a new numbering system.^[1] Tramways in Ballarat, Bendigo, Geelong, Sorrento and the Victorian Railways had their own tram numbering systems. Trams which were moved between the systems were renumbered.

There were also many non-passenger trams, such as rail grinders, breakdown cars and freight cars which were not included in the classification system.