Photograph: Pesda Press
A lateral buoy, lateral post or lateral mark, as defined by the International Association of Lighthouse Authorities, is a sea mark used in maritime pilotage to indicate the edge of a channel.
Each mark indicates the edge of the safe water channel in terms of port (left-hand) or starboard (right-hand). These directions are relative to the direction of buoyage; this is usually a nominally 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, it will be labelled on the appropriate chart.
A vessel heading in the direction of buoyage (eg into a harbour) and wishing to keep in the main channel should:
* keep port marks to its port (left), and
* keep starboard marks to its right.
For historical reasons, two different schemes are in use worldwide, differing in their use of colour.
Previously there had been 30 different buoyage systems, prior to IALA introducing the rationalised system.
The IALA defines them as System A and System B:
* System A is used by nations in Europe, Australia, New Zealand, parts of Africa and most of Asia other than the Philippines, Japan and Korea.
* System B is used by nations in North America, Central America and South America, the Philippines, Japan and Korea