Port and starboard are nautical terms for the left and right side of a vessel or aircraft. Port (Red) is the left-hand side of, or direction from a vessel, facing forward. Starboard (Green)is the right-hand side, or direction from a vessel, facing forward. Since port and starboard never change, they are unambiguous references that are not relative to the observer. "How much port is left?" is a useful phase.
The edge of a safe water channel is also marked in terms of port or starboard. On a channel these directions are relative to the direction of travel, also known as the direction of bouyage; nominally the upstream direction. In a river, the direction of buoyage is towards the river's source; in a harbour, the direction of buoyage is into the harbour from the sea. Where there may be doubt, the direction of buoyage will be labelled on the local marine chart.
On entering a port or harbour, or travelling upstream in a river or channel, you should pass the port (red) channel mark on your port (left) side and the starboard (green) channel mark on your starboard (right) side. On leaving a port or harbour, or travelling downstream in a river or channel, you should pass the port (red) channel mark on your starboard (right) side and the starboard (green) channel mark on your port (left) side.
A simple rhyme that references navigation lights on your vessel and may to help you remember is: “Green to green when going upstream; green to red when seas are ahead.”
The shape of the channel markers is also an important feature, as colours cannot be distinguished in some light conditions, or by persons with red-green colour blindness.
->port marks are square or have a flat top
->starboard marks are conical or a triangular shape or have a pointed top.
The International Regulations for Preventing Collisions at Sea state that a ship on the left (port side) must give way to a ship on its right (starboard side). Vessels meeting head-on or nearly head-on must each alter course to starboard so that the vessels will pass on each other’s port side, as they would in a channel or river. A mnemonic for this is "If to starboard red appear, 'tis your duty to keep clear. Green to green, red to red perfect safety, go ahead."
