Buddy diving just before and after the Second World War. After the invention of the "aqualung" by Cousteau and Gagnan, the first commercially underwater breathing apparatus became available for sale for sporting purposes in the late 1940s. As the new sport of scuba diving rapidly expanded through the 1950s, several sporting organisations—notably the YMCA—began programmes to train swimming enthusiasts in this new aquatic pastime, and began to codify what they believed were proper practises for this expanding sport. The YMCA considered the buddy system a useful corollary to the "never swim alone" rule of their swimming and lifesaving programmes. Cousteau himself independently
Death rates in the 20th century Death rates in the 20th century Death Rates in the 20th century is the ratio of deaths compared to the population around the world throughout the 20th century. When giving these ratios, they are most commonly expressed by number of deaths per 1,000 people per year. Many factors contribute to death rates such as cause of death, increasing the death rate, an aging population, which could increase and decrease the death rates by birth rates, and improvements in public health, decreasing the death rate. According to the CIA World Factbook, , the global crude death rate is 7.99 deaths/1,000 population.