A horizontal beam or spar that extends across a ship's mast perpendicular to it, typically for supporting rigging or sails.
From 'cross' (Old English 'cros') and 'tree' (Old English 'trēow', originally meaning 'wood'). The term dates to medieval nautical vocabulary when it referred to wooden structural elements.
A ship's crosstree was positioned at a critical weak point—right where the massive horizontal stress from sails and rigging tried to bend the mast sideways—so medieval shipwrights knew that crosstrees literally meant the difference between reaching port and snapping in half during a storm.
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