Most recreational scuba sets have a backup second-stage demand valve on a separate hose, a configuration called a "secondary", or "octopus" demand valve, "alternate air source", "safe secondary" or "safe-second". The idea was conceived by cave-diving pioneer Sheck Exley as a way for cave divers to share air while swimming single-file in a narrow tunnel, but has now become the standard in recreational diving. By providing a secondary demand valve the need to alternately breathe off the same mouthpiece when sharing air is eliminated. This reduces the stress on divers who are already in a stressful situation, and this in turn reduces air consumption during the rescue and frees the donor's hand.