regulator
a regulator (commonly depicted is the second stage regulator, that fits in the diver's mouth) is a basic piece of equipment for scuba and rebreather diving, and is divided into first stage, connected to the air cylinder, and second stage, and it's purpose is to supply the diver with air at a breathable pressure. it is common to find regulators with 2 different second stages, a primary mouthpiece and a backup mouthpiece in case of emergency (such as air sharing)