COLREG 4

Rule 12. 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:.
(i) 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,
(ii) 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;
(ii) 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 the
port or on the starboard side, she shall keep out of the
way of the other.
(b) For the purpose 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.