IALA Maritime Buoyage System

The IALA Maritime Buoyage System (IALA MBS) is the global standard for navigation marks at sea. It was unified in 1980 from earlier divergent systems and divides the world into two regions for the colour of lateral marks: Region A (most of the world — Europe, Africa, the Gulf, Asia, Australasia) uses red to port, green to starboard. Region B (the Americas, Japan, Korea, the Philippines) uses the opposite — green to port, red to starboard. The colour and shape of cardinal, isolated danger, safe water, and special marks are the same in both regions.

The direction of buoyage in a harbour is "from seaward" — when entering. On a chart it is shown by a magenta arrow. In coastal waters the direction generally runs clockwise around continents in the northern hemisphere and is given on the chart explicitly where it would otherwise be ambiguous.

Lateral marks — Region A

Port-hand mark (Region A)
Red light, any rhythm
Leave to port when entering harbour from seaward.
Starboard-hand mark (Region A)
Green light, any rhythm
Leave to starboard when entering harbour from seaward.

Lateral marks — Region B

Port-hand mark (Region B)
Green light, any rhythm
Leave to port when entering harbour from seaward ("red right returning" — opposite of Region A).
Starboard-hand mark (Region B)
Red light, any rhythm
Leave to starboard when entering harbour from seaward.

Preferred channel marks (Region A shown)

In Region B the colour scheme is reversed.

Preferred channel to starboard (Region A)
Red flashing 2+1 (Fl(2+1)R)
Bifurcation; preferred channel goes to starboard. Treat the mark as a port-hand mark of the preferred channel.
Preferred channel to port (Region A)
Green flashing 2+1 (Fl(2+1)G)
Bifurcation; preferred channel goes to port. Treat the mark as a starboard-hand mark of the preferred channel.

Cardinal marks

Topmarks are the most reliable cue: the points of the cones "point" toward the side that has black on the buoy. North = both up; South = both down; East = points apart (out); West = points together (in).

North cardinal
Q or VQ (continuous quick / very quick flashes), white
Pass to the north of the mark; safe water lies north.
East cardinal
Q(3) or VQ(3) every 10 / 5 s, white
Pass to the east of the mark; safe water lies east.
South cardinal
Q(6) + LFl every 15 s, or VQ(6) + LFl every 10 s, white
Pass to the south of the mark; safe water lies south.
West cardinal
Q(9) every 15 s or VQ(9) every 10 s, white
Pass to the west of the mark; safe water lies west.

Isolated danger, safe water, special, and emergency wreck

Isolated danger
White Fl(2)
Stationed on, moored to, or above an isolated danger of limited extent which has navigable water all around it.
Safe water
White isophase, occulting, LFl 10 s, or Morse "A"
Navigable water all around; mid-channel, fairway, or landfall mark.
Special mark
Yellow, any rhythm not used for white lights
Indicates a special area or feature whose nature is shown on the chart: spoil grounds, military exercise areas, cables and pipelines, ODAS and traffic separation marks where conventional buoys cannot be used.
Emergency wreck-marking
Alternating blue and yellow Q (1 s on, 0.5 s off), 200 m visibility minimum
Newly discovered hazard not yet shown on charts. Pillar or spar buoy, equal vertical blue and yellow stripes, yellow upright cross topmark.

Light rhythm shorthand

· Fl — single flash, single eclipse, with eclipse longer than flash.
· LFl — long flash, ≥ 2 seconds.
· Q — continuous quick flashing, 50–79 flashes per minute.
· VQ — continuous very quick flashing, 80–159 flashes per minute.
· Iso — isophase: equal duration of light and darkness.
· Oc — occulting: eclipse shorter than light period.
· Mo(A) — Morse-coded character (e.g. dot-dash for "A").
· Period (e.g. Q(3) 10 s) is the time for one complete sequence.