To retire at four-star grade, an officer must accumulate at least three years of satisfactory active-duty service in that grade, as determined by his or her service secretary. The President and Congress must also receive certification by either the Under Secretary of Defense for Personnel and Readiness, the Deputy Under Secretary of Defense for Personnel and Readiness, or the Secretary of Defense that the retiree served satisfactorily in grade. The Secretary of Defense may reduce this requirement to two years, but only if the officer is not being investigated for misconduct. The President may also reduce these requirements even further, or waive the requirements altogether, if he so chooses. Four-star officers who do not meet the service-in-grade requirement will revert to the next highest grade in which they served satisfactorily for at least six months which is normally the three-star grade. Since three-star ranks are also temporary, if the retiree is also not certified by the Secretary of Defense or the President to retire as a three-star, the retiree will retire at the last permanent rank he or she satisfactorily held for six months. The retiree may also be subject to congressional approval by the Senate before the retiree can retire in grade. It is extraordinarily rare for a four-star officer not to be certified to retire in grade or for the Senate to seek final approval.