Lateral marks define a channel and indicate the port and starboard sides of the navigation route to be followed into a waterway such as a harbour, river or estuary from seaward. The vessel should keep port marks to its left and keep starboard marks to its right.

If lateral marks are unable to be represented by a can or cone shaped buoy they should carry the appropriate topmark.

At the point where a channel divides, a modified lateral mark is used to indicate a ‘preferred’ channel (often a deep channel suitable for heavy commercial vessels) on one side and a secondary channel on the other. A preferred channel is indicated by red and green horizontal bands on the lateral marker. If a vessel wants to use the ‘preferred’ channel they observe the top colour of the mark while a vessel wishing to use the secondary channel observes the bottom colour.