What is "rule of thirds"

n. (context photography film visual arts English) A normative rule that an image should be mentally or representationally divided in three horizontally and diagonally, and that the focus of the image should not be in the center of the resulting shape.

The photograph to the right demonstrates the application of the rule of thirds. The horizon sits at the horizontal line dividing the lower third of the photo from the upper two-thirds. The tree sits at the intersection of two lines, sometimes called a power point or a crash point. Points of interest in the photo do not have to actually touch one of these lines to take advantage of the rule of thirds. For example, the brightest part of the sky near the horizon where the sun recently set does not fall directly on one of the lines, but does fall near the intersection of two of the lines, close enough to take advantage of the rule.

In scuba diving, the rule of thirds is a rule of thumb used by divers to plan dives so they have enough breathing gas remaining in their diving cylinder at the end of the dive to be able to complete the dive safely. 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.

For divers following the rule, one third of the gas supply is planned for the outward journey, one third is for the return journey and one third is a safety reserve. However, when diving with a buddy with a higher breathing rate or a different volume of gas, it may be necessary to set one third of your buddy's gas supply as your remaining 'third'. 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 he alone requires.

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. A diver without gas cannot do the stops and risks decompression sickness.

In an overhead environment, where it is not possible to ascend directly to the surface, the reserve allows the diver to donate gas to an out-of-gas buddy, providing enough gas to let both divers exit the enclosure and ascend to the surface.

Rule of thirds may refer to:

• Rule of thirds (diving)
• Rule of thirds, a photography rule of thumb
• Rule of thirds (tidal current rate)

The rule of thirds is a rule of thumb used in military organisation, according to which one third of the total military force should be available for operations, one third preparing to go on operations and the final third, having been on operations, in a period of recuperation. Ideally units and individuals will rotate through each of the three phases.