Fastening the regulator to the cylinder or cylinder block:

In an open-circuit scuba set, the first-stage of the regulator has an A-clamp, also known as a yoke, or a DIN fitting to connect it to the pillar valve of the diving cylinder. Yoke valves are the most common type by far; it clamps an open hole on the regulator against an open hole on the cylinder. This connection is made gas pressure tight with an O-ring. The DIN fitting is a type of direct screw-in connection to the cylinder. While less common worldwide, the DIN system has the advantage of withstanding greater pressure and is very common in central Europe.

Most yoke-type valves are of the K-valve, which is a simple on-off valve. In the mid-1960s, J-valves were widespread. J-valves contain a spring-operated shutoff that is triggered when tank pressure falls to 300-500 psi, causing breathing resistance and warning the diver that he or she is dangerously low on air. The reserve air is released by pulling a reserve lever on the valve. J-valves fell out of favor with the introduction of pressure gauges, which allow divers to keep track of their air underwater, especially as the valve-type are subject to accidental release of reserve air and increase the cost and servicing of the valve.