In modern scuba sets, a buoyancy compensator
(BC) or buoyancy control device (BCD), such as a back-mounted wing or
stabilizer jacket (otherwise known as a "stab jacket"), is built into the scuba
set harness. Although strictly speaking this is not a part of the breathing
apparatus, it is usually connected to the diver's air supply, in order to
provide easy inflation of the device. This can usually also be done manually via
a mouthpiece, in order to save air while on the surface.