Cardinal marks are used together with the compass to indicate where the mariner may find the best navigable water. They are placed in one of the four quadrants (North, south, east or west), boundered by inter-cardinal bearings. The mariner is safe if he passes north of a North cardinal mark, etc.

Uses.
Cardinal marks are used to:

1. Indicate that the deepest water in an area is on the named side of the mark;
2. Indicate the safe side on which to pass a danger;
3. Draw attention to a feature in a channel such as a bend, junction, bifurcation (where a channel splits into two) or end of a shoal.

Top marks.
Black, double-cone top marks are a very important feature of cardinal marks. They are carried with the cones as large as possible and clearly separated. The arrangement of the cones are as follows:

• North – The apexes of both cones point upwards.
• South – The apexes of both cones point downward.
• East – The apexes point away from each other, ie bases towards each other.
• West – The apexes point towards each other (looks like a wine glass).

Colours.
Black and yellow bands are used to colour cardinal marks. The position of the black band or bands is related to the points of the black top marks, ie

• North (Points up) – Black band above yellow band.
• South (Points down) – Black band below yellow band.
• East (Points outwards) – Black bands above and below yellow band.
• West (Points inwards) – Black band with yellow bands below and above.

Shape.
In the case of buoys they may be either pillar or spar.

Lights.
Cardinal marks use white light. Their characteristics are based on a group of quick or very quick flashes which distinguish them as cardinal mark and indicate their quadrant.

• North – Uninterrupted.
• East – 3 flashes in a group.
• South – 6 flashes in a group followed by a long flash.
• West – 9 flashes in a group.

To assist the memory, the number of flashes in each group can be associated with a clock face.

• East – 3 o’clock.
• South – 6 o’clock.
• West – 9 o’clock.

The long flash (not less than 2 seconds duration) immediately after the group of flashes of a south cardinal mark, is to ensure that it’s 6 flashes cannot be confused with a 3 or 9.

Retroreflectors.
One or more white bands, letters, numbers or symbols of retroreflective material are used in the standard code to distinguish unlit cardinal marks. Blue and yellow bands on the black and yellow parts of the mark are used in the comprehensive code.

• North – Blue on the black part and yellow on the yellow part.
• East – Two blue on the upper black part.
• South – Yellow on the yellow part blue on the black part.
• West – Two yellow on the upper yellow part.