In the early years of scuba, each diver carried a single second-stage regulator, and in the case of an out of air emergency, the buddy pair made an emergency ascent to the surface while the two divers took turns buddy breathing from the mouthpiece of remaining functional scuba set. Though this system worked effectively enough in a swimming pool or in open water practice sessions, and sometimes worked for skilled and disciplined divers in actual emergencies, in some cases stress and physical difficulties made it fail.