Port and starboard are nautical terms of orientation that deal unambiguously with the structure of vessels and aircraft, referring respectively to the left and right sides of the vessel, seen by an observer aboard the aircraft or vessel looking forward.
Vessel structures are largely bilaterally symmetrical, meaning they have mirror-image left and right halves if divided sagittally. One asymmetric feature is that on ships where access is at the side, this access is usually only provided on the port side.
Port and starboard unambiguously refer to the left and right side of the vessel, not the observer. That is, the port side of the vessel always refers to the same portion of the vessel's structure, and does not depend on which way the observer is facing.
The port side is the side of the vessel which is to the left of an observer aboard the vessel and facing the bow, that is, facing forward towards the direction the vehicle is heading when underway, and the starboard is to the right of such an observer.
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. The "steer-board" etymology is shared by the German Steuerbord, Dutch stuurboord and Swedish styrbord, which gave rise to the French tribord, Italian tribordo,[a] Catalan estribord, Portuguese estibordo and Spanish estribor.
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. In chapter 12 of Life on the Mississippi (1883) Mark Twain writes larboard was used to refer to the left side of the ship (Mississippi River steamboat) in his days on the river - circa 1857-1861.
An Anglo-Saxon record of a voyage by Ohthere of Hålogaland used the word "bæcbord" ("back-board") for the left side of a ship. With the steering rudder on the starboard side the man on the rudder had his back to the bagbord (Nordic for portside) side of the ship. The words for "port side" in other European languages, such as German Backbord, Dutch and Afrikaans bakboord, Swedish babord, Spanish babor, Portuguese bombordo, Italian babordo[a] and French bâbord, are derived from the same root.
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. To set forth these navigational rules, the terms starboard and port are 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. Aircraft 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.