The best alternative place to carry air source in scuba diving is to attach to the triangle area between your chin and your chest. Placing the alternative air source between the chin and the chest ensures that an out or air diver will be able to approach any diver underwater, locate and efficiently secure an alternate air source.
Other commonly acceptable positions to carry alternative air source are: attached to the Buoyancy Compensator (BC) on the Lower Left Hand side or Lower Right Hand Side and BC shoulder pocket.
Attached to the triangle area between your chin and your chest
The alternative air source is typically on a slightly longer hose than the primary in your mouth and is usually secured in sight of the triangle formed by our chin and the bottom of your rib cage in such a way that it pulls free with a firm tug.