<P> Three calendar systems are used by most American colleges and universities: quarter system, semester system, and trimester system . These are ways the calendar year is organized into a formal academic year, measured September--August or August--August . Some schools, particularly some business schools and community colleges, use the mini-mester or mini-semester system . </P> <P> The quarter system divides the calendar year into four quarters, three of which constitute a complete academic year . Quarters are typically 10--12 weeks long so that three quarters amount to 30--36 weeks of instruction . Approximately 20% of universities are on the quarter system . Most colleges that use the quarter system have a fall quarter from late September to mid-December, a winter quarter from early January to mid-March, a spring quarter from late March or early April to mid-June, and an optional summer session . Notable users of the quarter system include the University of California system (excluding Berkeley, Merced, the UCLA medical school, and all of the system's law schools), Stanford, the University of Chicago, Dartmouth College, Northwestern University, University of Washington, the University of Oregon, and DePaul University . Union College uses a modified quarter system: fall, winter, and spring terms are each eleven weeks long; there are some summer classes, but there is no official summer quarter . Because Union's calendar makes use of just three of four quarters, the three academic terms are generally (and somewhat misleadingly to outsiders) dubbed "trimesters," which they are not . Another notable and somewhat unique user of the quarter system is Baylor Law School: it operates on the quarter system while the remaining colleges of Baylor University operate on the semester system . </P> <P> The semester system divides the calendar year into two semesters of 16 to 18 weeks each, plus summer sessions of varying lengths . The two semesters together constitute 32 to 36 weeks of instruction, so that three academic quarters equal two academic semesters . Thus, academic credit earned in quarter hours converts to semester hours at 2⁄3 of its value, while credit earned in semester hours converts to quarter hours at 3 / 2 of its value . Put another way, 3 quarter hours is 2 semester hours . Most universities on the semester system have a fall semester from the day after Labor Day in September to mid-December, a spring / winter semester from late January to early May, and an optional summer session . </P> <P> In practice, the average quarter - long course is four or five units and the average semester course is three units, so a full - time student graduating in four years would take five courses per semester and three or four courses per quarter . </P>

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