A lateral buoy, lateral post or lateral mark, as defined by the International Association of Marine Aids to Navigation and 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. Often the cardinal mark system is used instead when confusion about the direction would be common.
A vessel heading in the direction of buoyage (e.g. into a harbour) and wishing to keep in the main channel should: