Other more complex arrangements of three or more cylinders are possible, but not usually of any advantage for recreational solo diving. Arrangements using H- and Y-valved back gas cylinders with two first stages may be useful if there is a significant risk of regulator freezing, but do not provide full redundancy. For solo diving an octopus regulator is not needed as it is intended for supplying gas to another diver. Another item which is very useful for the solo diver is the delayed surface marker buoy (DSMB) with a reel or spool suited to the depth of the planned dive. The primary function is to mark the position of the ascending diver, but it can also be used to control ascent rate, and to mitigate buoyancy loss malfunctions.