Crosstrees are the two horizontal struts at the upper ends of the topmasts of sailing ships, used to anchor the shrouds from the topgallant mast. Similarly, they may be mounted at the upper end of the topgallant to anchor the shrouds from the royal mast (if fitted). ==Explanation== Any vertical structure like a mast is subject to dynamic swaying s...
two horizontal struts at the upper ends of the topmasts of sailboats, used to anchor the shrouds from the topgallant mast.
two horizontal struts at the upper ends of the topmasts of sailboats, used to anchor the shrouds from the topgallant mast.
horizontal crosspieces at a masthead used to support ship's mast
horizontal crosspieces at a masthead used to support ship's mast
• (n. pl.) Pieces of timber at a masthead, to which are attached the upper shrouds. At the head of lower masts in large vessels, they support a semicircular platform called the `top.`
Cross'trees` noun plural (Nautical) Pieces of timber at a masthead, to which are attached the upper shrouds. At the head of lower masts in large vessels, they support a semicircular platform called the 'top.' 
Horizontal members attached to the mast acting as spreaders for the shrouds
[Treasure Island] two short bars across a ship's masthead to spread the rigging that supports the mast.