The procedure has been used since the beginnings of recreational diving, and along with the free ascent was one of the ways a diver could respond to running out of air at depth. At this time twin-hose regulators were the norm. and it was reasonably easy for two divers in a face-to-face position to share the regulator mouthpiece. Buddy breathing was an important skill before reserve valves and submersible pressure gauges were generally available, and running out of air was so common that it was not considered an emergency. By the mid-1960s the availability of submersible pressure gauges made reliable air supply monitoring possible and running out of air became less common.