Course (sail) on:
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In 
sailing
Sailing employs the wind—acting on sails, wingsails or kites—to propel a craft on the surface of the ''water'' (sailing ship, sailboat, raft, Windsurfing, windsurfer, or Kitesurfing, kitesurfer), on ''ice'' (iceboat) or on ''land'' (Land sa ...
, a course is a type of square sail
Square rig is a generic type of sail and rigging arrangement in which a sailing vessel's primary driving sails are carried on horizontal spars that are perpendicular (or square) to the median plane of the keel and masts of the vessel. These sp ...
. It is the sail set on the lowest yard
The yard (symbol: yd) is an English units, English unit of length in both the British imperial units, imperial and US United States customary units, customary systems of measurement equalling 3 foot (unit), feet or 36 inches. Sinc ...
on a mast. The courses are given a name derived from the mast on which they are set, so the course on the foremast may be called the fore-course or the foresail; similarly main-course or mainsail for that carried on the mainmast. On the mizzen, a course is not usually carried. If it is, it is called the crossjack (or cro'jack) or mizzen sail. The lower yard on the mizzen takes the same name (crossjack). When there is no sail set from this yard it is still needed to sheet home the topsail. In that case, a clue to the arrangement is the absence of footropes.
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