Alternate Air Sources
Diving became a lot safer with the introduction of alternate air sources. These devices allow you to share air with your buddy in the event of an out-of-air emergency. The three common alternate air sources are octopus regulators, integrated air sources, and redundant scuba systems.
No matter which option you use, it's important to place it at or near your chest so your buddy can easily locate and retrieve it when needed. Clips are available that hold the air source's mouthpiece secure, but allow your buddy to quickly pull it free when needed.
An octopus is an additional 2nd stage that is connected to your 1st stage, and is the most common alternate air source used today. With an octopus, both you and your buddy can breathe from your cylinder while ascending.
The octopus works like any other 2nd stage, but most have a couple minor modifications. The most important is a longer hose that allows it to reach your buddy. Another common modification is a yellow cover to make it more visible when clipped to your chest. Some, like the lower example to the left, have a low-profile design to decrease their size.
The octopus is popular because it's easy to use and costs less than other options. But keep in mind that when using the octopus, your air supply will be depleted faster because two divers are sharing the same air source.
Integrated Air Sources
Another alternate air source that's gaining popularity is the integrated air source. This is a BCD power inflator with a fully functional 2nd stage built into it. This option is popular because there are no extra hoses or equipment to deal with.
In the event of an out-of-air emergency, the donor breathes from the integrated air source while the buddy breathes from the donor's primary 2nd stage. This procedure is different than what most divers are familiar with, so always be sure your buddy understands this air sharing procedure before beginning the dive.
Like the octopus, integrated air sources share one air source between divers, so be sure to monitor your air supply when sharing air.
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.