Rule of thirds is a practice followed by cave divers who gauge the maximum distance they are able to penetrate into a cave system. The breathing gas they carry with them is divided into three or thirds. The first third of gas is used while penetrating deep into the cave. When a third of the gas is consumed, this signals the diver it is time to begin exiting the cave system. The thought is the second third of the gas will be adequate for exiting the system while leaving the remaining third of the gas for an emergency.
