Solo diving II. In the late 1940s, after the invention of the Aqua-lung by Cousteau and Gagnan, the first retail underwater breathing apparatus for sport was commercially marketed. As the sport expanded through the 1950s, several sporting organisations – notably the Young Men's Christian Association (YMCA) – began scuba training programmes for swimming enthusiasts, thus began the codification of what was believed to be proper practices for the expanding amateur sport of scuba diving. The buddy system was thought to be a useful corollary to the "never swim alone" edicts of the YMCA swimming and lifesaving programmes. Cousteau independently implemented a buddy
Aqua-Lung than its precursor, which in fact was almost completely unknown outside France. In addition, and most importantly, the Aqua-Lung could be mounted on stronger and reliable air tanks holding up to 200 atmospheres, allowing extension of diving duration to more than an hour at significant depths (including the needed time for decompression stops). The first Cousteau-Gagnan Aqua-Lungs (like the CG45 of 1945 or the Mistral of 1955) were mainly twin-hose open-circuit scuba. They have since been made by various manufacturers with varying design details and numbers of cylinders. Like modern open-circuit scuba with single-hose regulators, they consisted of one or