[1][2] Different nations and militaries will vary the rule of thirds according to their own financial and manpower situations.
A rule of halves was actually the basis of British Army planning during the British Empire,[citation needed] with each regiment consisting of a pair of battalions which would take turns recruiting, training and recuperating in the UK, and then being deployed overseas.
However, because of the nature of transportation in the past, each of the phases at that time were longer (being a year or more) than is currently the case.
Departing from a three-phase rotation for short periods is not impossible, but prolonged deployment is detrimental both to the psychological health of service personnel, and to the operational life of equipment, leading to an unwanted turnover in personnel, and to premature failure of equipment.
[citation needed] However, the United States is one of the few nations that can afford both the manpower costs of doing this, while still being able to ensure that enough boats are in service to have the ability regularly to stagger boats out for deep maintenance without appreciably affecting the total number available.