the medium tank (the introduction of M60). The MBT would form the backbone of modern ground forces. Many Cold War MBTs evolved more or less directly from late World War II medium tank designs. However, in the 1960s and 1970s, a generation of purpose-designed main battle tanks appeared, starting with the British Chieftain tank. These vehicles are less obviously influenced by wartime templates (the Chieftain, for example), weighing as much as a World War II heavy tank and possessing far greater firepower and armour, while retaining the mobility of the previous Centurion design. Similarly, the US M1 Abrams series, the German Leopard 2, the British Challenger 1, French Leclerc and Russian T-90 tanks are all main battle tanks. The defining feature of the main battle tank type is neither its weight, mobility, nor firepower, but instead the idea that only one type of tracked armoured vehicle is required to carry out the roles of breakthrough, exploitation and infantry support. Specialist tank Tanks have often been modified for special purposes. The most common is armoured recovery vehicles, used during combat for recovery or repair of battle-damaged and inoperable armoured fighting vehicles. Another common use is to provide armoured capability for combat engineers. These include tanks carrying large-calibre demolition guns, with flails or ploughs for mine-clearing, or flame tanks armed with flamethrowers. The tank occasionally may lose its weapons and the chassis alone may be used, as in bridge-laying tanks. Another important modification was the amphibious tank. These designs were modified with waterproofing and propulsion systems, to be able to traverse open water. Many specialist tank roles have been assigned to other vehicle types, though many tank chassis are still used for a wide variety of vehicles, ranging from anti-aircraft roles to bridge layers. Unmodified tanks can be fitted with equipment, such as mine-clearing ploughs, to give them ancillary roles. Hobart's Funnies were a group of various specialist tanks used in World War II, named after Major General Percy Hobart. Tank generations Tanks are sometimes classified as belonging to a particular generation, although the actual definition and membership in these generations is not clearly defined. Soviet and Russian military planners organise tanks into a generation of tanks up to 1945, and four generations of main battle tanks, while Canadian strategists organise main battle tanks into three generations. The military of the People's Republic of China also recognises three generations of its own