Technical divers may need to carry several different gas mixtures. These are intended to be used at different stages of the planned dive profile, and for safety reasons it is necessary for the diver to be able to check which gas is in use at any given depth and time, and to open and close the supply valves when required, so the gases are generally carried in fully self-contained independent scuba sets, which are suspended from the harness at the diver's sides. This arrangement is known as stage mounting. Stage sets may be cached along a penetration guideline to be retrieved during exit for convenience. These are also sometimes called drop tanks.