4.2 U.S. Aids to Navigation: Lateral Marks (IALA-B)
Key Takeaways
- The United States uses IALA Region B ("red right returning"): keep red marks to starboard when returning from seaward or proceeding in the conventional direction of buoyage.
- Red lateral marks are even-numbered; green lateral marks are odd-numbered, increasing as you return from seaward.
- Preferred-channel (junction) marks are red-and-green horizontally banded; the top color shows the preferred side.
- Dayboards and buoy shapes support color: red nuns/triangles vs green cans/squares on the standard U.S. system.
- Lighted lateral aids use red lights on red marks and green lights on green marks so color matches the aid’s lateral meaning at night.
Lateral Marks Guide You Through the Channel
Aids to navigation (ATON) mark safe water, channels, hazards, and special areas. Lateral marks tell you which side of the channel is which when you are following the conventional direction of buoyage. For U.S. waters that direction is almost always returning from seaward — entering harbor from the ocean, or proceeding upstream on many rivers as the system defines. Q162 hammers lateral rules because a Master who confuses red and green puts a loaded passenger boat on the wrong side of the channel.
IALA Region B — Americas Convention
The International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA) defines two buoyage regions:
| Region | Where (high level) | Returning from seaward |
|---|---|---|
| IALA-A | Much of Europe, Africa, Asia, Australia, etc. | Green to starboard |
| IALA-B | Americas, Japan, Korea, Philippines, etc. | Red to starboard |
The United States uses IALA Region B. The memory phrase every U.S. captain learns is:
Red Right Returning
When returning from seaward (or following the conventional direction of buoyage inland):
- Keep red lateral marks to starboard (right)
- Keep green lateral marks to port (left)
When leaving harbor for sea, reverse the picture: red is to port, green to starboard as you head out — but the marks did not change; your direction relative to “returning” changed. Exam questions often set the scene as “entering port” or “returning from seaward” so you apply red-right without overthinking.
Do not apply Region A habits on a U.S. Master exam. “Green right returning” is the European pattern and is a deliberate distractor.
Numbering: Even Red, Odd Green
U.S. lateral marks carry numbers (sometimes with letters) that increase as you return from seaward:
- Red lateral marks → even numbers (2, 4, 6…)
- Green lateral marks → odd numbers (1, 3, 5…)
If you are heading into port and see red “6” then red “8,” you are progressing inbound along the starboard side of the channel system. Numbers help you match chart labels to real-world buoys when visibility is poor or several aids look similar.
On ICW and some waterways you will also see yellow squares, triangles, or bands as dual-purpose or ICW markers overlaying the lateral system — learn the local convention from the chart and Coast Pilot, but the base red/green lateral rule still organizes the main channel colors.
Shapes and Dayboards
Color is primary; shape reinforces identity by day:
| Side (returning) | Color | Typical buoy shape | Typical dayboard |
|---|---|---|---|
| Starboard | Red | Nun (conical top) | Triangle (red) |
| Port | Green | Can (cylindrical) | Square (green) |
Beacons (fixed structures) use dayboards with the same color/shape language so you can read them without a floating hull. Unlighted daybeacons still count as lateral ATON when they mark the channel edge.
Preferred-Channel (Junction) Marks
Where a channel splits, a preferred-channel mark (junction buoy/beacon) uses horizontal bands of red and green. The top band color is the color of the preferred channel’s “hand” when returning from seaward:
- Red band on top → treat like a red lateral mark for the preferred route (leave it to starboard when returning).
- Green band on top → treat like a green lateral mark for the preferred route (leave it to port when returning).
The secondary channel is the other branch; it may still be usable, but the top color marks the primary route for strangers and for most exam scenarios. Daybeacons use the same red-green banding logic with matching dayboard shapes.
Light Colors Match the Lateral Mark
At night, red lights mark the red/starboard-hand side when returning; green lights mark the green/port-hand side. Light rhythm (flashing, quick, occulting, etc.) distinguishes one aid from another; light color carries the lateral meaning. You do not expect a green light on a red nun as a normal lateral setup.
Characteristics (period, flash pattern, nominal range) come from the Light List and chart abbreviations — covered in the next section — but lateral identity is still red vs green first.
Ranges, Leading Lights, and Channel Alignment
Some channels use range lights (two lights or daymarks in line) rather than only floating laterals. When the front and rear range marks align, you are on the centerline of the channel. Lateral buoys still mark the sides; ranges mark the track. On a Master 100 night approach, if the range opens left or right, you are drifting off the recommended line even if GPS still looks “in the channel.” Exam items may pair ranges with lateral color questions — answer both systems correctly.
Conventional Direction on Rivers and the ICW
Not every waterway’s “return from seaward” is obvious. On many rivers buoyage increases toward the head of navigation (upstream as defined for that system). On the Intracoastal Waterway, yellow squares, triangles, or bands identify the ICW route in addition to red/green lateral colors for the local channel system. When the exam gives a river or ICW stem, read the question’s direction cue and the chart’s buoyage notes. When the exam simply says “returning to port from sea,” apply red right returning without inventing local exceptions.
Typical trap: a candidate applies European IALA-A memory (“green right returning”) because of overseas time or a bad mnemonic app. On ONC06, U.S. waters are Region B unless the stem explicitly places you abroad — and this Master 100 guide’s Navigation General module is Near Coastal U.S. practice.
Lateral Marks and the Small-Passenger Master
On a Master 100 boat you often run the same inlet weekly. Complacency kills: after a storm, a nun may be off station, dragged, unlit, or replaced by a temporary buoy of different rhythm. Rules of good practice:
- Compare what you see to the charted position, number, and Light List entry
- Report extinguished or off-station aids to the Coast Guard and check Local Notice to Mariners
- Do not cut corners inside green cans at night on a falling tide just because “we always do”
- Brief relief skippers on any temporary lateral changes before they take the next charter
Lateral ATON is a language. Fluency means you can reverse the picture when outbound, identify preferred channel at a fork, and still know which side is “right” when only a single flashing red is visible through rain.
Lateral system pocket card
- U.S. = IALA-B = Red Right Returning
- Red even / green odd, numbers increase inbound
- Nun/triangle red; can/square green
- Preferred channel = red-green horizontal bands; top color = preferred hand
- Light color matches lateral color; ranges mark the track line
Pass Q162 lateral items cold and you also pass the night approach when the GPS track looks fine but the first green can is suddenly on the wrong side of your bow — that is when Region B fluency earns its keep.
In U.S. waters under IALA Region B, which statement is correct when returning from seaward?
How are standard U.S. red and green lateral marks numbered?
At night, what light color should you expect on a standard red starboard-hand lateral buoy when returning from seaward?