Although they were two distinct pieces of the whole Apollo spacecraft, the Command and Service Modules were heavily interdependent vehicles that provided life support and functionality to the Apollo spacecraft. At the very top of the spacecraft, the Command Module (CM) functioned both as living and working quarters for the astronauts, containing all of the equipment and instruments necessary to operate the spacecraft. The CM was attached to the Service Module, which supplied power, life support, and propulsion to the CM. One module could not function properly without the other, a fact that was amply demonstrated during the Apollo 13 mission, when a malfunction on the Service Module forced the astronauts to rely on the Lunar Module’s life support for the journey back to Earth. The Command and Service Modules were built by North American Rockwell.