<P> The trimester system evolved out of the semester system . It divides the academic year into three equal portions of 15--16 weeks each . Institutions that use the trimester system include California Institute of Technology, Carleton College, Knox College (Illinois), Lawrence University, and the United States Merchant Marine Academy . The fall and winter trimesters constitute an academic year of 30--32 weeks . The spring / summer trimester is usually divided into a 7.5 week Spring Session followed by a 6 - week or 7.5 week Summer Session . The reduced maximum course load that accompanies the shortening from the traditional semester makes the trimester system compatible with the semester system . Academic credit is thus measured on the trimester system in semester hours; there is no such thing as a "trimester hour" of credit . </P> <P> At the University of Michigan and Brigham Young University, for example, the Fall trimester (informally still called' semester') operates from September through December; the Winter trimester runs from January through April; and the spring - summer trimester operates from May through August, as two half - trimesters . Most spring - summer classes either meet double - time for 7--8 weeks in May and June or double - time / double - plus - time for 6--8 weeks in July and August (with summer half - term classes sometimes starting in the last week of June). </P> <P> The quinmester academic calendar at Park University operates with five terms per year, each lasting eight weeks (January--March, March--May, June--July, August--October, and October--December). </P> <P> A number of colleges have adopted the "one course at a time" or "block schedule" calendar . Academic years consist of a number of terms lasting roughly four weeks each, during which a full semester's amount of work is completed in one and only one class . Colorado College first began their "Block Plan" in 1970, followed by Maharishi International University in 1971, and Cornell College in 1978 . Quest University in Squamish, British Columbia; Tusculum College in Tusculum, Tennessee; and The University of Montana - Western are the only other colleges operating under this academic calendar . </P>

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