United States military seniority is the method by which the United States Armed Forces determines precedence among commissioned officers, in particular those who hold the same rank. Seniority is used to determine assignments, tactical commands, promotions and general courtesy. To a lesser extent, historical seniority is used to recognize status of honor given to early United States military leaders such as inaugural holders of certain ranks or those officers who served as leadership during major wars and armed conflicts. Modern-day seniority The modern-day seniority system of the United States commissioned officer corps operates on two different levels. For officers of different ranks, seniority is simply determined by who holds the highest rank. For instance, Army colonel is senior to captain and captain senior to lieutenant. Seniority extends across services as for instance major in the Army is senior to captain in the Air Force while commander in the United States Navy is senior to both. For officers in the same rank or paygrade, seniority is determined by the dates on which they assumed their ranks. If officers of the same grade have the same date of rank, then seniority is determined in order by the officer's previous rank's date and so forth. If all promotion dates of ranks are the same, seniority is then determined on order of: previous active duty grade relative seniority (if applicable), total active commissioned service, and finally, total federal commissioned service or date of appointment as a commissioned officer. The secretaries of each service may establish further seniority rules if applicable. Whenever there are two or more officers in the same grade or rank, who are on duty in the same area, field command, or organization, and who are all eligible to command, the president may assign command to an officer, without regard to time-in-rank. A type of "positional seniority" exists for military officers who hold top leadership positions of the armed forces. For instance, the Chairman of the Joint Chiefs of Staff is considered the most senior officer of the entire United States military, even though it is possible that contemporaries of the same rank may have earlier dates of rank or longer time in service. Likewise, heads of various armed service branches are considered most senior within their service; unified commanders are also considered most senior in their respective regions yet not necessarily to each other. The regular United States military hierarchy is