The double carabiner is well-known among SCUBA divers and is used to attach all kinds of equipment to the jacket. It has proven its worth millions of times and can be easily operated, even with gloves.

The double carabiner is also useful in freediving: you can use it to secure a line to a spool or a reel, preventing the line from unwinding further. Simply wrap the line twice around one of the carabiner hooks and hook the other carabiner somewhere.