In the military, vehicles such as trucks or tractor units can be used to transport or launch missiles (rockets with warheads), essentially a form of rocket artillery. Such a vehicle may transport one or multiple missiles. The missile vehicle may be a self-propelled unit or the missile holder/launcher may be on a trailer towed by a prime mover. They are used in the military forces of a number of countries in the world. Long missiles are commonly transported parallel to the ground on these vehicles, but elevated into an inclined or vertical position for launching. Missile vehicles include transporter erector launchers (TEL) and multiple rocket launchers (MRL) such as the Patriot missile system. Single or dual missile vehicles often transport their missiles uncovered. The missile batteries of multiple rocket launchers often hold their missiles inside tubular or rectangular canisters for each missile, from which the missiles or rockets can be launched (or fired, in other words). Many missile trucks use pneumatic (air-filled) tires, although they may be large and specialized for offroad travel. However, some missile vehicles use tractor crawler drive similar to that of a tank.