Tanks are often referred to by weight-based classifications such as 'light', 'medium' or 'heavy', and by extension the role that this size of tank was suitable for. There were many names given to different tank types, and similar names did not assure similar design goals. Some light tanks were relatively slow, and some were fast. Some heavy tanks had large-calibre, low-velocity, anti-infantry bunker-busters, and some had high-velocity anti-tank guns. Furthermore, expected weights for a given tank type vary over time; a medium tank of 1939 could weigh less than a light tank of 1945.