Senior officers typically retire well in advance of the statutory age and service limits, so as not to impede the upward career mobility of their juniors. Since there are a finite number of four-star slots available to each service, typically one officer must leave office before another can be promoted. Maintaining a four-star rank is like a game of musical chairs; once an officer vacates a position bearing that rank, he or she has no more than 60 days to be appointed or reappointed to a position of equal or greater importance before he or she must involuntarily retire. Historically, officers leaving four-star positions were allowed to revert to their permanent two-star ranks to mark time in lesser jobs until statutory retirement, but now such officers are expected to retire immediately to avoid obstructing the promotion flow.