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A trysail is small fore-and-aft sail used in very high winds or in storms to maintain control, to avoid ship damage, and to keep the bow to the wind. It is hoisted abaft the mainmast (taking the place of the much larger mainsail).
Sleight, Steve (1999). The Complete Sailing Manual. ISBN 0-7894-4606-5,
Sails spars and rigging