Although most universities in the United States are on a semester system, which offers classes in the fall and spring, some schools observe a quarter system comprised of fall, winter, spring and summer quarters. The academic year, September to June, is divided into three quarters of eleven weeks each beginning in September, January, and March; the summer quarter, June to August, is composed of shorter sessions of varying length. Students may take advantage of the opportunity to study year round by enrolling in all four quarters. Most students begin their programs in the fall quarter, but they may enter at the beginning of any of the other quarters.