until the multi-role concept evolved into the main battle tank, rendering the earlier medium and heavy designations obsolete. Heavy tanks were largely withdrawn from service as medium multi-role vehicles offered similar capability with less of the weight-based constraints. Light tanks remained in use for flexibility, such as with air-portable use. Size classifications 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. While originally based on weight, the light, medium, and heavy classifications expanded based on tactical use. They now have other meanings than just weight, including relation to gun size, the amount of armour, and, most importantly, tactical role. Post-war in 1948 France, Canada, and the United States agreed to classify tanks as light gun, medium gun, or heavy gun. After World War II, less expensive armoured cars and more specialised tracked vehicles gradually took over the roles of light tanks. Heavy tanks were shown to be incapable of keeping up with mobile warfare, but advances in engine, weapon, and armour technology allowed medium tanks to acquire the best characteristics of heavy tanks, allowing them to fulfil multiple roles on the battlefield. The ultimate in mobility, firepower, and protection were rolled into the main battle tank (MBT). In 1957, the Fourth Tripartite Armour Conference recommended to replace medium and heavy tanks with a single class — Main Battle Tanks. World War I In World War I, the first tank, the British Mark I, was designed for supporting infantry by crossing trenches and attacking machine-gun posts. This became known as a heavy tank alongside other, lighter, types. A lighter British tank introduced into service in 1918, at and armed with machine guns only was given the designation "Tank, Medium Mark A" and known as the "Whippet". The two-man French Renault FT was known as a light tank. Super-heavy breakthrough tanks such as the Char 2C () or the K-Wagen () were nearly completed before the war