Surface swimming (also known by its acronym, SF) is swimming on the surface of water using goggles or mask, snorkel and monofins. SF races are held for distances of 50, 100, 200, 400, 800, 1500, 4 × 100 relay and 4 × 200 relay (metres) in swimming pools and over various long distances in the open water environment. Swimmers must remain on the surface of the water at all times for the duration of the race except when starting or make a turns at the end of a swimming pool where an immersion over a distance of 15m is permitted. Long distance surface swimming include races in open water, such as 6000m and 4×3000 relay.