In sailing, a course sail is the principal sail on a mast.

This term is used predominantly on square rigged vessels, referring to the largest and lowest sail on each mast (mizzen-course, main-course and fore-course). Gaff-rigged vessels might possibly use the term (for the lowest sail rigged aft of each mast), but are more likely to refer simply to a mainsail, foresail, etc. A Bermuda- or lateen-rigged yacht, whether sloop, cutter, ketch or yawl, would not usually be described as having a course.

sailing vessels and rigging