should align themselves to allow convenient movement of the demand valve from donor to recipient and back. When the divers need to ascend or swim horizontally, it requires co-ordination and some skill, which is best practiced in a low hazard environment. Training A study by the UCLA Diving Safety Research Project suggests that about 20 successful repetitions of buddy breathing during training of entry level students are needed for a reasonable expectation of success without errors, and retesting after three months without reinforcing practice showed degraded performance and procedural errors. This is a higher level of training than provided by most recreational diver training organisations. For the skill to be reliable in an emergency, periodic reinforcement is necessary, and familiarisation is particularly valuable when buddies are to dive together for the first time. Hazards Buddy breathing is one of the few scuba skills in which the incompetence of one diver can threaten the life of another. Poor buddy breathing performance has resulted in the death of both divers on more than one occasion. History Buddy breathing originated from military diving following a prohibition on the training and practice of free ascents. 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. In the late 1960s single-hose regulators started to take over as the standard, and this complicated the buddy breathing procedure. The single hose exhaust valve position at the bottom of most DVs made it necessary to offset the recipient to the right side when face to face, or to the left when side by side. The standard procedure of continuous exhalation during ascent to avoid lung over-pressure injury could leave the diver with insufficient air to clear the regulator, so it was necessary to hold the DV in a way that did not obstruct the recipient from