Scuba diving is an underwater diving mode where divers use breathing equipment completely independent of a surface breathing gas supply, and therefore has a limited but variable endurance. The word scuba is an acronym for "self-contained underwater breathing apparatus" and was coined by Christian J. Lambertsen in a patent submitted in 1952. Scuba divers carry their source of breathing gas, affording them greater independence and movement than surface-supplied divers, and more time underwater than freedivers. Although compressed air is commonly used, other gas blends are also employed.