ColRegs Rule 14: Head-on Situation
(a) When two power-driven vessels are meeting on reciprocal or nearly reciprocal courses so as to involve risk of collision each shall alter her course to starboard so that each shall pass on the port side of the other. (b) Such a situation shall be deemed to exist when a vessel sees the other ahead or nearly ahead and by night she would see the mast head lights of the other in a line or nearly in a line and or both sidelights and by day she observes the corresponding aspect of the other vessel. (c) When a vessel is in any doubt as to whether such a situation exists she shall assume that it does exist and act accordingly.
(From Nautical Rules of the Road – ColRegs for power boating and sailing – a Safe-Skipper App)