Freedivers traditionally dived alone or with an attendant on the surface to assist with the harvest, and many of the early diving bells were only large enough for a single occupant. When surface supplied diving was first developed, it was common to have only one diver unless the work required more. Those early traditions continue and customarily include a standby diver, and a working diver who is in constant communication with the surface control crew. The sport of scuba diving is rooted in a multitude of small enthusiastic snorkelling and spearfishing clubs that date back to the decades just before and after World War 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 system after a number of harrowing diving incidents that date back to the earliest days of exploratory diving. The buddy system's useful aspects have long been established, including the cross-checking of equipment before dives, the facilitating of assistance for possible entanglement problems or equipment failures, and enhancement of the social nature of diving. The YMCA was a substantial influence in the development of diver certification during the first 50 years of the sport. As various scuba programmes were adopted by emerging scuba certification agencies, such as the National Association of Underwater Instructors (NAUI), Professional Association of Diving Instructors (PADI), and British Sub-Aqua Club (BSAC), the practise of buddy diving inspired one of the two main mantras of recreational scuba: "never hold your breath" and "never dive alone".