Port and starboard are nautical and aeronautical terms of orientation that deal unambiguously with the structure of vessels and aircraft. Their structures are largely bilaterally symmetrical, meaning they have mirror-image left and right halves if divided long-ways down the middle.
One asymmetric feature is that on aircraft and ships where access is at the side, this access is usually only provided on the port side.
To understand which is which, when a person is facing the bow on a vessel or aircraft, that is, forward towards the direction the vehicle is heading when underway, the port side is the left-hand side and the starboard side is the right-hand side. However, port and starboard never change; they are unambiguous references that are not based on the relative directions of an observer, just as the cardinal directions of east and west do not change no matter the direction a person faces.
The term starboard derives from the Old English steorbord, meaning the side on which the ship is steered. Before ships had rudders on their centrelines, they were steered with a steering oar at the stern of the ship on the right hand side of the ship, because more people are right-handed. Since the steering oar was on the right side of the boat, it would tie up at the wharf on the other side. Hence the left side was called port. The Oxford English Dictionary cites port in this usage since 1543.
Formerly, larboard was often used instead of port. This is from Middle English ladebord and the term lade is related to the modern load.Larboard sounds similar to starboard and in 1844 the Royal Navy ordered that port be used instead. The United States Navy followed suit in 1846.Larboard continued to be used well into the 1850s by whalers.
The navigational treaty convention, the International Regulations for Preventing Collisions at Sea--for instance, as appears in the UK's Merchant Shipping (Distress Signals and Prevention of Collisions) Regulations 1996 (and comparable US documents from the US Coast Guard)--sets forth requirements for maritime vessels to avoid collisions, whether by sail or powered, and whether a vessel is overtaking, approaching head-on, or crossing.:11-12 To set forth these navigational rules, the terms starboard and port are absolutely essential, and to aid in in situ decision-making, the two sides of each vessel are marked, dusk to dawn, by navigation lights, the vessel's starboard side by green and its port side by red.:15Aircraft are lit in the same way.
An order, recently issued by the Lords Commissioners of the Admiralty, states, that in order to prevent mistakes, which frequently occur from the similarity of the words starboard and larboard, in future, the word port is to be substituted for larboard, in all Her Majesty's ships or vessels.