[1][2] This rule generally only applies to diving in overhead environments, such as caves and wrecks, where a direct ascent to the surface is impossible and the divers must return the way they came.
This means that the turn point to exit is earlier, or that the diver with the lower breathing rate carries a larger volume of gas than they alone require.
Reserves are needed at the end of dives in case the diver has gone deeper or longer than planned and must remain underwater to do decompression stops before being able to ascend safely to the surface.
If all goes to plan, the divers will surface with stages and primary cylinders each containing about one third of the original content.
[3] With the rule of thirds, the duration of the dive is limited by the point at which the gas reaches 1/3 the starting quantity, by not exceeding the planned decompression obligation, and by returning along the same route in similar conditions.