At night, the port side of a boat or aircraft is indicated by a red navigation light at the bow. The opposite side of the bow has a green one to help avoid collisions. The "International Regulations for Preventing Collisions at Sea" state that a ship on the left must give way to a ship on its right. If the courses of two boats are intersecting, the helmsman usually gives way to a red light by going around the stern of the stand-on vessel (the vessel that does not need to change course). A saying for this is: