The basic principle of breathing gas management is never to run out of suitable breathing gas while underwater. The implications depend on the team size and the dive profile, and range from very simple for a situation where the diver can make a safe free ascent at any point of the dive, to complex, when a team of two or three divers is constrained from surfacing immediately by decompression obligations or a hard overhead, and rely on each other for emergency gas supply at any point of the planned dive, particularly when a variety of gases are carried which are each only suitable for a specific depth range.