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In more detail, a regulator in SCUBA terms consists of a primary (or first) stage regulator, atleast one secondary (or second) stage regulator, and usually 4 hoses.

The primary stage attaches directly to the high pressure air tank and feeds high pressure and low pressure hoses. The high pressure hose usually goes to your dive computer or gauges, the low pressure hoses go to your BCD and secondary stage regulator(s). The low presure hoses contain air at the same pressure as the water arround you.

The secondary stage regulators take the low pressure feed from the primary and supply you with air in a controled manner. (eg only when you breathe in). You usually have a primary second stage (the one you normaly breathe through), and a backup second stage, (aka the Octi).