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 direction of safe water in terms of port (left-hand) or starboard (right-hand). These directions are relative not only to the mark but also to the direction from which the mark is viewed; the interpretation is thus defined in terms of a vessel approaching in a nominally upstream direction. In a river, upstream is taken as leading towards the river's source; in a harbour, whether or not the harbour is fed by a river, a vessel entering the harbour is taken as moving upstream.

The interpretation of the marks is thus that a vessel heading upstream or entering a harbour:

• keeps a port mark to its port (left), and
• keeps a starboard mark to its right.

Marks are distinguished by their colour, being red or green, and shape. For historical reasons, two different schemes are in use worldwide, differing in their use of colour. The IALA defines them as System A and System B:

• System B is used by nations in North America, Central America and South America, the Philippines, Japan and Korea.
• 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.

In System A:

• port marks are red and may have a red flashing light.
• starboard marks are green and may have a green flashing light.

(Port) (left) (Red) _____________________________________ (Starboard) (right) (Green)

In System B:

• port marks are green, and may have a green flashing light.
• starboard marks are red and may have a red flashing light.

(Port) (left) (Green) _____________________________________ (Starboard) (right) (Red)

In both systems:

• port marks are square or have a flat top
• starboard marks are conical (or present a triangular shape) or have a pointed top.

The shape is an important feature, as colours cannot be distinguished in some conditions of direct sunlight, and by persons with red-green colour blindness. Marks may also carry unique markings of letters and numbers; these may be used to identify the mark as one indicated on a nautical chart. Likewise, a mark's light may flash in a distinctive sequence for the same purpose.

Also defined in the lateral mark systems is a bifurcation mark, which has both red and green horizontal bands, one over the other. This mark indicates that a "preferred" channel (often, a deep channel suitable for heavy commercial traffic) lies to one side, and a secondary channel on the other. Vessels wishing to use the preferred channel observe the top colour of the mark, and vessels wishing to use the secondary channel observe the bottom colour.

In System B only, the phrase "red right returning" may be used as a mnemonic, indicating that a red mark must be kept on the right when returning to (i.e., entering) a harbour or river.

The alternative system of cardinal marks is used where there is no clear upstream reference direction.

External link

• A web site of Transport Canada (http://www.tc.gc.ca/marinesafety/Tp/tp14070/b3-buoys.htm), showing lateral marks in System B.