2nd Stage

The second stage is the part of the regulator you are most familiar with; as its always in your mouth.  It has an important job to provide the diver air at the ambient pressure (surrounding pressure); without this function, it would be very hard to breathe underwater.  The primary (main) and secondary (spare/octopus) second stages are identical in function, and can be interchangeable; just remember you may regret saving money on your octopus in an emergency!