A square-rigged ship has sails that are raised and lowered with a horizontal spar. As the name implies, square-rigged sails have four corners; the sails on modern fore-and-aft rigged ships are triangular. While the operation of a traditional three-masted square-rigger requires the management of hundreds of ropes and lines, the basic elements of navigating one of these ships are fairly easy to understand.
Run downwind by setting the spars, or "yards," at a 90-degree angle to the keel line of the ship; the sheets should be made fast at an equal length at either side of the spars. The yards in this position are said to be "squared." The rudder should be kept straight ahead, adjusted slightly from time to time to keep the wind dead astern.
Sail at an angle downwind by using the sheets to pull the yards in the opposite direction of that in which you want the ship to head. So, if the wind is at your back and you want to head to port, you swing the spars over to starboard. In this case, the sheet on the starboard side would be made fast at a shorter length than the sheet on the port side. The yards in this position are said to be "braced." The rudder may need to be held slightly to the leeward side to resist the ship's tendency to head straight downwind.
Tack (change the direction of the ship to the angle of the wind) by releasing the sheets and bringing the yards over to the opposite angle relative to the keel, and bracing them in the opposite direction. The rudder is turned in the direction of the tack.
Tack into the wind by bracing the yards over in the direction you want to head. The rudder must be kept slightly to windward to keep the vessel from falling off to a 90-degree angle to the wind.
Square-rigged ships are very efficient for running downwind, which was typical for most long ocean voyages. They set much more sail directly against a wind coming from astern than a fore-and-aft rig can. You can get a taste of sailing a square-rigged ship by booking a cruise on a tall ship. Most of these cruises give passengers the opportunity to stand a watch at the wheel and haul on the sheets.
The closest to the wind that a square-rigger can sail is 70 degrees off (as opposed to just 40 degrees the fore-and-aft ships can attain). For this reason, when the wind is blowing from the general direction in which you want to head, you'll have to sail great distances in tacking in order to make any forward progress. Square-rigged ships are much more difficult to turn entirely around (or "bring about") than a fore-and-aft rigged ship. They have a greater tendency to lose momentum when passing through the eye of the wind, a humiliating situation known as being "in irons."