The Lateral System: Red and Green Markers

The Foundation Rule: "Red, Right, Returning"

The lateral system is the most common and most important part of the U.S. ATON system. It is designed to mark the sides (lateral edges) of navigable channels, guiding you between hazards to keep you in safe water.

The entire lateral system is built on one simple, foundational mnemonic:

"Red, Right, Returning"

This means:

• When you are proceeding in the "returning from sea" direction...
• You keep the red-colored aids on your right (starboard) side...
• And therefore, you keep the green-colored aids on your left (port) side.

If you are going in the opposite direction (heading out to sea, "departing"), you simply reverse the rule:

• Keep red aids on your left (port) side
• Keep green aids on your right (starboard) side

This simple rule is the key to staying within the safe, marked channel and avoiding running aground.

The area between the red and green markers is the navigable channel-water with adequate depth and freedom from obstructions. The areas outside the markers (beyond the reds on one side and beyond the greens on the other) contain hazards: shallow water, rocks, sandbars, or other dangers.

What Does "Returning from Sea" Mean?

The most common source of confusion for new boaters is understanding what "returning from sea" actually means. "Returning" is a defined term in navigation (officially known as the Conventional Direction of Buoyage), and it does NOT mean "returning to your specific dock or marina."

"Returning from sea" is defined as follows:

Coastal Waters

Proceeding from a large body of water into a smaller body of water:

• From the ocean into a harbor
• From a bay into a river
• From the sea into an inlet or sound

Example: Entering San Francisco Bay from the Pacific Ocean = returning from sea. The red markers will be on your starboard (right) side as you enter the bay.

Rivers and Inland Waterways

Proceeding upstream (against the current):

• Following a river toward its source
• Moving against the predominant flow of water

Example: Traveling up the Hudson River from New York Harbor toward Albany = returning from sea. Red markers on starboard.

Great Lakes

The "returning" direction on the Great Lakes is generally defined as:

• Proceeding north
• Proceeding west

This convention treats the Great Lakes as if they "flow" from west and north toward the St. Lawrence Seaway outlet to the east.

Intracoastal Waterway (ICW)

The ICW has a special defined direction (covered in detail in Lesson 4):

• Southbound on the Atlantic Coast
• Westbound along the Gulf Coast

This creates a "clockwise" conventional direction around the United States.

The Foolproof Check: The Numbering System

If you are ever disoriented and cannot determine which direction is "returning from sea," there is an absolutely foolproof method: look at the buoy numbers.

In the lateral system, marker numbers always increase (ascend) as you proceed in the "returning from sea" direction.

Practical Application:

Scenario 1: You are in a channel and pass a marker numbered "7". The next marker you reach is numbered "9". The numbers are increasing (7 → 9). You are definitively proceeding in the returning direction. Keep red markers to starboard, green to port.

Scenario 2: You pass a marker numbered "8", then the next one is "6". The numbers are decreasing (8 → 6). You are definitively heading out to sea. Keep red markers to port, green to starboard.

The Elevator Analogy: If you get in an elevator and the floor numbers are increasing (3... 5... 7...), are you going up to the penthouse or down to the lobby? Obviously up. Buoy numbers work exactly the same way-increasing numbers = "going up" the waterway (returning).

This numbering system provides absolute certainty about direction, eliminating any guesswork.

Port-Side Markers (The "Green" Side)

When you are "returning from sea," the port (left) side of the channel is marked by aids with the following characteristics:

Color: Green

All green aids mark the port side when returning. The color may be a bright kelly green or a darker forest green depending on age and manufacturer, but it's unambiguously green (not blue-green or teal).

Number: Odd numbers (1, 3, 5, 7, 9, 11...)

The number is displayed in white on the green background. Numbers increase as you proceed in the returning direction.

Buoy Shape: "Can" buoy

Can buoys are cylindrical with a flat top. They look like a floating tin can or drum. The flat top is the distinctive identifying feature visible from a distance.

Dayboard Shape: Square

If the aid is a fixed beacon (daybeacon), it will display a square green dayboard with the odd number in white.

Light Color (if lighted): Green

If the aid has a light, the light will be green. Common light rhythms include:

• Flashing (Fl) - regular flash
• Quick Flashing (Q) - rapid flashing
• Isophase (Iso) - equal light and dark periods

Mnemonic: "Green Cans and Green Squares are on your left when you've left the sea."

Additional mnemonic: "ODD and LEFT both have fewer letters than their opposites" (both are 3 letters vs. EVEN-4, RIGHT-5).

Starboard-Side Markers (The "Red" Side)

When you are "returning from sea," the starboard (right) side of the channel is marked by aids with the following characteristics:

Color: Red

All red aids mark the starboard side when returning. The red may range from bright red to darker brownish-red with age and weathering, but is distinctly red.

Number: Even numbers (2, 4, 6, 8, 10, 12...)

The number is displayed in white on the red background. Numbers increase as you proceed in the returning direction.

Buoy Shape: "Nun" buoy

Nun buoys are conical (cone-shaped) with a pointed top. The name "nun" comes from the resemblance to the traditional pointed habit (head covering) worn by certain orders of nuns.

Dayboard Shape: Triangle

If the aid is a fixed beacon, it will display a triangular red dayboard with the even number in white. The triangle typically points upward.

Light Color (if lighted): Red

If the aid has a light, the light will be red. Common light rhythms are the same as green lights (Fl, Q, Iso).

Mnemonic: "Even Nuns Blush Red"

This mnemonic reminds you: Even numbers, Nun shape, Red color.

Another mnemonic: "Red Right Returning" (the foundational rule itself)

Buoy Shapes: Why Are Nuns and Cans Different?

The distinct shapes of can and nun buoys serve a critical safety function: shape provides independent confirmation when color alone may be ambiguous.

Real-world scenario: You're navigating at dawn, dusk, or into the glare of the setting sun. Color perception is severely degraded. A faded red buoy and a faded green buoy may look nearly identical in color-both appearing as dark, indistinct shapes.

However: You can always distinguish between:

• A flat-topped cylinder (can)
• A pointed-top cone (nun)

The shape alone tells you which side of the channel the marker indicates, even when color is invisible or ambiguous.

This is why dayboards are also different shapes: A square (port) looks completely different from a triangle (starboard) at distance, even in poor visibility or when colors have faded.

Safety principle: Always confirm both color AND shape. If you see a red cone (nun), you have double confirmation it's a starboard marker. If you see green but the shape is pointed (impossible by rule), you have a damaged or improperly maintained aid-report it and navigate with extreme caution.

Practical Application: Staying in the Channel

Let's consolidate this into clear, actionable navigation guidance:

When RETURNING from sea (numbers increasing, going upstream, entering harbor):

Keep RED Nuns / RED Triangles on your RIGHT (Starboard)

Keep GREEN Cans / GREEN Squares on your LEFT (Port)

Navigate in the water BETWEEN them

When HEADING OUT to sea (numbers decreasing, going downstream, leaving harbor):

Keep RED Nuns / RED Triangles on your LEFT (Port)

Keep GREEN Cans / GREEN Squares on your RIGHT (Starboard)

Navigate in the water BETWEEN them

Critical safety rule: Do NOT pass on the "wrong side" of a lateral marker. These marks are positioned at the edge of safe water. Passing outside them (beyond the greens on the port side, or beyond the reds on the starboard side) puts you in shallow water or over obstructions.

The "favoring" technique: In unfamiliar channels or areas with uncertain depths, "favor" (stay closer to) the side with more frequent markers. If red markers are closely spaced but green markers are far apart, the reds are marking a more critical edge (shallow water or hazards closer to that side).

Junction Markers (Preferred Channel Markers)

Channels don't always run in simple straight lines. Sometimes a channel splits-a secondary channel branches off, or the main channel divides around an island or shoal.

The ATON system uses special junction markers (also called preferred channel markers) to mark these channel splits.

Key characteristics:

Color: Horizontal bands of red and green

These aids display BOTH red and green in horizontal stripes, indicating they mark a junction where either side can be navigated.

Topmost band: Indicates the preferred (main) channel

The color of the topmost band indicates which side the marker should be treated as for navigating the preferred channel.

Preferred Channel to Starboard (Green-Red-Green bands)

Top band: Green

Meaning: The preferred (main, deepest, best-marked) channel is to starboard of this marker

How to navigate: Treat the buoy as if it were entirely green (a port-side marker)

• Keep it on your port (left) side
• The preferred channel passes to the right (starboard) of the marker

Buoy shape (if floating): Can (flat top) - because it's treated as a green/port marker

Dayboard shape (if fixed): Square - same reason

Mnemonic: "Green on Top, Keep it to Port"

Preferred Channel to Port (Red-Green-Red bands)

Top band: Red

Meaning: The preferred (main, deepest, best-marked) channel is to port of this marker

How to navigate: Treat the buoy as if it were entirely red (a starboard-side marker)

• Keep it on your starboard (right) side
• The preferred channel passes to the left (port) of the marker

Buoy shape (if floating): Nun (pointed top) - because it's treated as a red/starboard marker

Dayboard shape (if fixed): Triangle - same reason

Mnemonic: "Red on Top, Give it a Starboard Slot"

Can You Use the Non-Preferred Channel?

Yes, both channels at a junction are navigable, or the junction marker wouldn't be there. The "non-preferred" channel may be:

• Narrower
• Shallower (but still navigable for appropriate draft vessels)
• Less well-marked
• Subject to shoaling or seasonal depth changes

When to use non-preferred channel:

• Your destination is in that direction
• Traffic in preferred channel is heavy
• Local knowledge indicates it's safe for your vessel draft
• Chart shows adequate depth for your vessel

Safety consideration: When in doubt, follow the preferred channel (main/red/green buoy indication). It will have better depth, better markers, and is less likely to present unexpected hazards.

Summary

The lateral system uses red and green markers to indicate channel edges. The foundational rule "Red, Right, Returning" means keep red markers to starboard when proceeding from sea into harbors, up rivers, or in the defined "returning" direction.

Green markers (odd numbers, can shape, square dayboards) mark the port side when returning. Red markers (even numbers, nun shape, triangular dayboards) mark the starboard side. Marker numbers always increase in the returning direction, providing foolproof directional confirmation.

Junction markers use horizontal red/green bands, with the topmost color indicating the preferred channel direction. Treat junction markers as if they were the color of their top band.

Shape confirmation (can vs. nun, square vs. triangle) provides critical backup when color is ambiguous due to lighting, weather, or fading. Always navigate between the markers, never beyond them into hazard areas.

Key Takeaways

• "Red, Right, Returning" is the foundation rule for lateral navigation
• "Returning" = from sea into harbor, upstream in rivers, north/west on Great Lakes, defined direction on ICW
• Numbers ALWAYS increase when returning-foolproof directional check
• Green = odd numbers, can shape, square dayboard, port side
• Red = even numbers, nun shape, triangular dayboard, starboard side
• Shape provides independent confirmation when color is ambiguous
• Junction markers have horizontal bands-top color shows preferred channel
• Green-top junction: keep to port (preferred channel to starboard)
• Red-top junction: keep to starboard (preferred channel to port)
• Navigate BETWEEN markers, never pass beyond them into hazard areas