From Nicholas McLaren,
Your Guide to Scuba Diving.
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Definition: The Alternate Air Source is your backup second stage regulator. It works just the same as the primary second stage regulator and can be used in the unlikely event that your primary fails or your dive buddy needs to share air. It is usually a bright color such as fluorescent yellow or pink and is normally secured somewhere within the triangle between the chin and the bottom of the rib cages, normally on the right side.

Also Known As: Octopus