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
Scuba set ("Self-Contained Underwater Breathing Apparatus"). Lambertsen's invention, for which he held several patents registered from 1940 to 1989, was a rebreather and is different from the open-circuit diving regulator and diving cylinder assemblies also commonly referred to as scuba. Open-circuit-demand scuba is a 1943 invention by the Frenchmen Émile Gagnan and Jacques-Yves Cousteau, but in the English language Lambertsen's acronym has become common usage and the name "Aqua-Lung", (often spelled "aqualung"), coined by Cousteau for use in English-speaking countries, has fallen into secondary use. As with radar, the acronym "scuba" has become so familiar that it is generally not capitalized and