College courses are assigned a value in what are called "credits" or "units." The number of units assigned to a course corresponds to the number of hours that a student will attend class for that course. For example, a course that consists of three class sessions per week, and where each class session last for 50 minutes, will be assigned a value of three units. Typically, colleges require that students complete a minimum number of units in order to graduate, rather than a minimum number of courses. This gives students more flexibility in what courses they decide to take to complete their graduation requirements.
Most colleges and universities follow either a quarter-based calendar system or a semester-based calendar system. In a quarter system, the academic year is divided into three sessions called quarters. Each quarter lasts about 12 weeks. There is usually an additional quarter in the summer, where registration is optional. Foreign students are not required to attend courses in the summer to maintain their status. In a semester system, the academic year is divided into two sessions called semesters. Each semester lasts 16 weeks. Again, there may be an optional session during the summer.
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Students who have registered for at least 12 units in a session (either a quarter or semester) are said to be "full-time" students. Students who have registered for fewer than 12 units in a session are called "part-time" students. Foreign students must maintain a "full-time course load" (in other words, they must always register for at least 12 units) in order to maintain their visa status.