A straight piece of timber or a hollow cylinder of wood or metal set up vertically or nearly so and supporting yards, booms, derricks, or gaffs. In fore-and-aft rigged vessels each mast is commonly made of two parts, called the lower mast and the topmast. On large sailing vessels the masts are made up of several lengths called lower mast, topmast, topgallant mast, and royal mast. Masts are supported from forwards by stays, from sides by shrouds (rigging), and from aft by backstays.