To simplify the procedure for air sharing, the recreational diving industry moved to a configuration that provided each diver an additional second-stage regulator, as a backup to the primary. The backup is known variously as the "octopus stage", "backup", "secondary", or (obsolescent) "safe second". The term "octopus" came about because several regulator and other hoses hanging from the first stage made the unit look a bit like an octopus. Two general systems have evolved for carrying and deploying the backup demand valve—one more prevalent in recreational diving and the other commonly found in technical diving (although some crossover exists). In both systems, each diver carries two demand valves. They may be attached to the first-stage regulator of a single tank or to two first stage regulators of twin cylinders. The primary regulator is for normal breathing during the dive, and the secondary regulator ("octopus") is a backup for oneself or an out-of-air buddy.