Square rig

Square rig is a generic type of sail and rigging arrangement in which the primary driving sails are carried on horizontal spars which are perpendicular, or square, to the keel of the vessel and to the masts. These spars are called yards and their tips, outside the lifts, are called the yardarms. A ship mainly rigged so is called a square-rigger.

In 'Jackspeak' (Royal Navy slang) it also refers to the dress uniform of Junior Ratings.