Seaman's Handbook

PART B. Steering and Sailing Rules : Sailing vessels

(a) When two sailing vessels are approaching one another, so as to involve risk of collision,
one of them shall keep out of the way of the other as follows:
  1. when each has the wind on a different side, the vessel which has the wind on the port
    side shall keep out of the way of the other;
  2. when both have the wind on the same side, the vessel which is to windward shall keep
    out of the way of the vessel which is to leeward;
  3. if a vessel with the wind on the port side sees a vessel to windward and cannot
    determine with certainty whether the other vessel has the wind on her port or starboard
    side, she shall keep out of the way of the other.
(b) For the purposes of this rule the windward side shall be deemed to be the side opposite to that on which the mainsail is carried or, in the case of a square-rigged vessel, the side opposite to that on which the largest fore-and-aft sail is carried.