A yard is a spar on a mast from which sails are set. It may be constructed of timber or steel or from more modern materials such as aluminium or carbon fibre. Although some types of fore and aft rigs have yards, the term is usually used to describe the horizontal spars used on square rigged sails. In addition, for some decades after square sails were generally dispensed with, some yards were retained for deploying wireless (radio) aerials and signal flags.

Note that these terms refer to stretches of the same spar, not to separate component parts.

The yard can rotate around the mast to allow the direction of the vessel to be changed relative to the wind. When running directly downwind the yards are 'squared' (pointing perpendicular to the ship's centre line and to the mast). As the ship is steered closer to the wind the yards are braced round using the braces. When further rotation is obstructed by other bits of rigging (typically the shrouds), the yard is said to be braced "hard round" or "sharp up", as in "sharp up to port". This angle (normally about 60 degrees) limits how close to the wind a square rigged ship can sail.

The yards represent a considerable weight high above the vessel's centre of gravity; to increase stability, especially in heavy weather, some means is normally provided to lower some of the yards when they are not being used to set sails. In nineteenth-century warships (where a large crew was available) this was generally by physically "sending down" the upper yards from the masts and storing them on deck—along with, in many cases, the upper sections of the mast itself. Merchant ships in the age of sail would also do this before sailing in the Southern Ocean. On modern tall ships the yards are not designed to be sent down on deck, but 'lifting yards' that can be raised and lowered along a short section of mast using a halyard are often used.

In order to set and stow the square sails, the crew must climb aloft and spread out along the yards. To do this, they stand in footropes suspended beneath the yard and balance themselves between that and the yard itself. The person working on the end of the yardarm has a separate footrope known as the flemish horse. Jackstays run along the top of the yard. The sail will be bent on to one of them, but a second one is often provided (particularly on larger yards) for the crew to hold on to. These are usually steel rods, but stiff cordage stretched between the yardarms was used in the past.

Almost all ships used in modern times are fitted with steel safety wires (sometimes erroneously called jackstays) along the yards to which sailors attach themselves using a harness. This is a relatively recent innovation. Cargo-carrying and naval sailing ships were not so equipped and falling from the yard represented a real risk.

The yard exists to allow square sails to be set to drive the ship. The top edge of the sail is 'bent on' (attached) to the yard semi-permanently. Clewlines and buntlines are led along the yard and from there to the mast and down to the deck. These allow the bottom of the sail to be hoisted up to the yard, so the sail is effectively folded in two. In this state the sail is said to be "in its gear", that is ready for setting or stowing. To set the sail the clewlines and buntlines are let go, and the sheets (attached to the bottom corners, or clews, of the sail) are adjusted to shape the sail to best catch the wind. A device called a "tye", a "tye chain", or a "tye rope" is used for lifting a given yard to the top of its travel to set the sail.

To stop using (i.e., to "hand") the sail, the sheets are released and the clewlines and buntlines are pulled tight. The sail folds in half—back in its gear—and so no longer catches the wind. Unless the sail is to be used again very soon, the next step is to "stow" it. To do this, the crew must go out along the yard in order to bundle the sail up tightly and tie it down with gaskets.