<P> 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 . </P> <P> 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 . </P> <P> A vessel heading in the direction of buoyage (e.g. into a harbour) and wishing to keep in the main channel should: </P>

What is the main purpose of the lateral system of red and green buoys and markers