Redundant scuba, or a pony bottle, consists of a small cylinder with its own regulator, and is commonly referred to as a pony bottle. The cylinder may be strapped to the diver's primary cylinder, or clipped to the BC.

These systems are heavier and more expensive than other options, but have major advantages. With redundant scuba, you are not dependent on your buddy in the event you run out of air. It also allows you to share air with your buddy without sacrificing your own air supply. For these reasons, this system preferred for deep dives.