Gas planning where there is no direct way up: one third in, one third out, one third held in reserve. The turn pressure is fixed before you enter the water.
With no direct route to the surface, \"I'll just keep an eye on the gauge\" stops being a plan. The rule of thirds turns your reserve into a number fixed before you get in the water: one third in, one third out, one third left untouched. What matters is what that last third is for — not you alone, but the case where two divers are breathing from one supply. For more open profiles, tech divers also calculate minimum gas, often called rock bottom: the amount two divers together need from the deepest point to the first gas switch or the surface. Beginners run sixths rather than thirds while their own consumption under stress is still hard to judge. The usual failure is not the arithmetic but the discipline: with good visibility and a good mood, turn pressure quietly gets stretched. Write it on your slate beforehand — stress ruins mental maths.