A sloop is a sailboat usually with one mast and fore-and-aft rigged sails. It can also describe a square rigged sailing ship of two or more masts which is sometimes called a sloop of war. The French called them corvettes. Originally the definition of a sloop was vague. A 1750 book said sloops "are sailed and masted as men's fancies lead them, sometimes with one mast, with two, and with three." Modern recreational sloops are almost always one-masted sailing ships with fore-and-aft sails. Their simple layout makes them popular with amateur sailors. The sloop is a manageable sailboat layout up to a length of about .