prior to receiving their commission typically don't make it to the 40 years of commissioned service mark, because they are still subject to the age restrictions for retirement. For example, Admiral Michael G. Mullen was born on 4 October 1946; placed on active duty in 1968 and promoted to admiral on 23 August 2003. Ordinarily, he would have been expected to retire at the end of his four-year term as chief of naval operations in 2008 after 40 years of service. Instead, he was reappointed as an admiral and appointed as chairman of the Joint Chiefs of Staff on 1 October 2007. He retired from the Navy after serving two, two-year terms as chairman on 1 October 2011, at the age of 65 with 43 years of service and eight years in grade. General James F. Amos was born on 12 November 1946; placed on active duty in 1970 and promoted to general on 3 July 2008. Ordinarily, he would have been expected to retire at the end of his two-year term as assistant commandant of the Marine Corps in 2010 after 40 years of service. Instead, he was reappointed as a general and appointed as commandant of the Marine Corps on 22 October 2010. He retired from the Marine Corps after completing his four-year term as commandant on 17 October 2014, at the age of 67 with 44 years of service and six years in grade. General Frank J. Grass was born on 19 May 1951; enlisted in the Missouri Army National Guard in October 1969 and received his commission in 1981. He was appointed as a general in the active duty reserves and assigned as chief of the National Guard Bureau on 7 September 2012. He remained on reserve active duty until he completed his four-year term as chief and retired from the Army on 3 August 2016, at 65 years of age with 35 years in commissioned service, 47 years of total service, and four years in grade. 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