Second stage valve:

Direct feed or power inflator:

A connection to inflate a buoyancy compensator or a drysuit, is manually operated by a button or lever or knob.

Demand valve:

This type of second stage is called demand valve or DV. It is fed by a medium pressure hose from the first-stage. It works as described in the #Types of last stage section above. When the diver breathes out, the air goes to the dry side of the diaphragm, and is released to the outside through (usually two) one-way valves. It has a purge button, which the diver can press to depress the diaphragm to make gas flow to blow water out of the mouthpiece (or for other purposes such as filling a lifting bag).

Octopus:

Sometimes (nowadays nearly always) a single-hose regulator has more than one demand valve (= DV). If the extra DV is simply a spare DV for use by the diver's buddy it is usually called an octopus. The medium pressure hose on the octopus is usually longer than the medium pressure hose on the DV that the diver uses.

Combined DV and BC inflator:

The demand valve could be a hybrid DV and buoyancy compensator inflation valve. Both types are sometimes called alternate air sources, and more confusingly a DV on a regulator connected to a separate independent diving cylinder would also be called an "alternate air source".

Full face mask:

There have been at least two cases of a single-hose-type demand regulator last stage built into a circular fullface mask so that the mask's big circular front window plus the flexible rubber seal joining it to its frame, was a very big and thus very sensitive regulator diaphragm:-

• A version of the Le Prieur breathing set. Yves Le Prieur patented it in 1946 and the patent was granted on 10 February 1947.
• Captain Trevor Hampton invented independently a similar regulator-mask in the 1950s and submitted it for patent, but the Royal Navy requisitioned the patent, but found no use for it and eventually released it, but by then the market had moved on and it was too late to make this regulator-mask in bulk for sale.

Dive/surface valve or bailout valve: