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triangular sail that was of decisive importance to medieval navigation. The ancient square sail permitted sailing only before the wind; the lateen was the earliest fore-and-aft sail. The triangular sail was affixed to a long yard or crossbar, mounted at its middle to the top of the mast and angled to extend aft far above the mast and forward down nearly to the deck. The sail, its free corner secured near the stern, was capable of taking the wind on either side, and, by enabling the vessel to tack into the wind, the lateen immensely increased the potential of the sailing ship.