A multistage (or multi-stage) rocket is, like any rocket, propelled by the recoil pressure of the burning gases it emits as it burns fuel. What characterizes it as "multi-stage" is that it successively jettisons one or more stages as they become empty. It is effectively one or more rockets (stages) stacked on top of or attached next to each other ("parallel staging"); in order to reduce the total amount of mass which needs to be accelerated to the final speed/height. Generally each stage consists of one or more motors, plus fuel and oxidiser tanks for a liquid rocket or the casing for a solid rocket. In rocketry, this concept is known as staging. Solid or liquid rocket Boosters are often used for parallel staging schemes and all motors are ignited at launch.