The most prevalent method for masks intended for surface supply applications is known as a bailout valve. The surface supplied diver generally carries a bailout gas supply sufficient to reach a place of safety based on the planned dive profile. This gas supply is regulated by a scuba first stage regulator, and plumbed into the primary air supply circuit via a bailout valve before it reaches the demand valve. The full face masks designed for surface supply work usually mount this valve on the side of the mask frame, in the bailout block, where it is convenient and easily reached by the diver. The bailout cylinder valve is normally open during the dive. To switch to bailout, the diver simply opens the bailout valve, and is immediately supplied by gas from the bailout cylinder.