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.
Variations and implications
Different nations and militaries will vary the rule of thirds according to their own financial and manpower situations.
At times of financial constraint rather than increase the total number of forces one temptation of politicians is to attempt increase available manpower by folding together the preparation and recuperation phases, creating a rule of halves. A rule of halves was actually the basis of British Army planning during the British Empire, each regiment consisting of a pair of battalions which would take turns recruiting/training/recuperating in the UK and being deployed overseas, however because of the nature of transport each of the phases at that time were longer (being a year or more) than is currently the case. Departing from a 3 phase rotation for short periods is not impossible but prolonged departure is detrimental to both the psychological health of service personnel and the operational life of equipment, leading to an unwanted turnover in personnel. For those who can afford it, going to a four or five phase rotation actually increases a nation's ability to conduct sustained military operations, even though it may appear that the majority of its servicemen are not doing anything.
As a critical cornerstone of their defence policies the British and French submarine-launched ballistic missile forces uses a rule of fourths, one submarine on patrol, one preparing to go on patrol, one having returned from patrol, and the fourth in maintenance. This force structure ensures that they will always have at least one ballistic missile boat on patrol.
The United States, in order to keep the maximum number of its submarines available at any one time, assigns two crews, called gold and blue, to each of its submarines, with the submarines themselves having a turnaround time that is as short as possible. However the United States is one of the few nations that can both afford the manpower costs of doing this while having enough boats in service to stagger boats out for deep maintenance without appreciably affecting the total number available.