The United States uses the IALA Maritime Buoyage System Region B — IALA-B — for all federally maintained aids to navigation. The most important fact about this system is also the fact most likely to catch an international visitor off guard: it is the reverse of the system used in Europe, South America, Africa, Asia, and most of the rest of the world. In IALA-A, the red marks are on the left when returning from sea. In IALA-B, they are on the right. "Red right returning" is the US mnemonic, and it has kept millions of mariners in the channel. If you have sailed in European waters and are now sailing in US waters — or vice versa — this is not a trivial distinction. Review it before you approach any unfamiliar harbor.
Lateral Marks: The Red and Green System
Lateral marks define the sides of a navigable channel. In IALA-B, when you are proceeding from seaward toward the head of navigation — returning from sea — the red marks are on your starboard (right) side and the green marks are on your port (left) side. This is true whether you are entering a harbor, proceeding up a river, or transiting any marked channel. The direction of "returning" is generally defined as moving from the open water toward the harbor, up a river from its mouth, or in the conventional direction of navigation on a specific waterway (the USCG defines this explicitly for the Intracoastal Waterway and other major routes).
Red nun buoys are conical — they taper to a point at the top, resembling a nun's habit. They carry even numbers (2, 4, 6...). When maintained with a light, the light is red. They mark the right side of the channel when returning.
Green can buoys are cylindrical, flat-topped, and carry odd numbers (1, 3, 5...). When maintained with a light, the light is green. They mark the left side of the channel when returning.
Numbers increase from seaward to harbor. If you are entering a channel and the buoy numbers are increasing, you are heading in the conventional direction of travel. Buoy 2 is closer to open water; Buoy 24 is deeper into the harbor or up the river. This numbering convention is a secondary check: if you are uncertain which direction is "returning from sea," the increasing numbers tell you which way the conventional traffic flows.
Dayboards — fixed markers on pilings rather than floating buoys — follow the same color and numbering convention. A red triangular dayboard marks the right side of the channel; a green square dayboard marks the left.
Light Characteristics: Decoding the Flash Pattern
Lighted aids flash in specific patterns that allow identification at night or in reduced visibility, and that distinguish them from background lights ashore. The USCG Light List — published annually in seven volumes covering different regions of the US coast — provides the complete characteristics of every federally maintained light. Charts show abbreviated versions of the same data. Here are the standard light characteristics you will encounter:
Flashing (Fl) — single flash at regular intervals, with the period of darkness longer than the period of light. The most common characteristic for a straightforward lateral mark. "Fl R 4s" means a red light that flashes once every four seconds.
Quick (Q) — a rapid series of flashes, 60 per minute or more. Quick flashing lights are used at important turns or high-traffic junctions where a rapidly visible identification is needed. "Q G" is a quick green light.
Isophase (Iso) — equal periods of light and dark. An isophase light spends exactly as much time on as off. Used on some preferred channel marks and entrance lights.
Occulting (Oc) — the light is on more than it is off; the characteristic is defined by the darkness, not the flash. An occulting light that goes dark once per cycle is "Oc." Less common than flashing, but you will encounter it.
Group flashing (Fl(2), Fl(3)) — a specified number of flashes in a group, repeated at intervals. "Fl(2) R 6s" means two red flashes in close sequence, then a pause, the whole cycle taking six seconds. Group flashing lights are used at specific locations where a unique signature is needed to distinguish one aid from nearby aids.
The period shown in the Light List (and on the chart) is the time in seconds for one complete cycle — from the start of one flash or group to the start of the next. A light with "Fl G 2.5s" completes its cycle in 2.5 seconds. Counting the period with a watch is a reliable way to confirm identification when you are uncertain which mark you are looking at.
Preferred Channel Marks: Junction Buoys
Where a channel splits into two navigable branches, a preferred channel mark indicates which branch is the primary (preferred) channel. These buoys carry both red and green horizontal bands. The color of the topmost band tells you which is the preferred side.
If the topmost band is red, the preferred channel is to starboard — treat the mark as if it were a red lateral mark and keep it on your right when returning. The light, if any, is red with a composite group-flash characteristic: Fl(2+1) R.
If the topmost band is green, the preferred channel is to port — treat the mark as a green lateral mark and keep it on your left when returning. The light characteristic is Fl(2+1) G.
You can pass a junction buoy on either side — the channel is navigable both ways — but the preferred channel is deeper, better maintained, or more suitable for larger vessels. In shoal water or with a deep draft, default to the preferred channel unless you have local knowledge that the secondary channel has adequate depth.
Safe Water Marks: All Clear Around
Safe water marks indicate that the water is navigable on all sides of the mark — there are no hazards immediately adjacent. They are not part of the lateral system; they do not indicate a side of a channel. They are typically found at harbor entrances, at the seaward end of a channel, or in mid-channel where a specific reference point is useful.
Safe water marks are red and white vertical stripes — the stripes run vertically up the buoy, alternating red and white. They are spherical, or carry a red spherical topmark. When lighted, the light is white and the characteristic is either isophase (Iso W), occulting (Oc W), or a single long flash every ten seconds (Fl(1) 10s W). The "Mo(A)" characteristic — Morse code letter A, a short flash followed by a long flash — is also used on safe water marks in some locations.
A safe water mark at a harbor entrance is telling you that the approach is clear. Once you pass it, you follow the lateral marks into the channel.
Range Lights: The Centerline Indicator
Range lights are two lights positioned so that when you are on the correct track — the centerline of the channel — the two lights appear vertically aligned. The front (near) light is lower; the rear (far) light is higher. When they are stacked vertically, you are on the range line. When the rear light appears to the left of the front light, you have drifted to the right of the centerline. When the rear light is to the right of the front light, you have drifted left.
The correction is always in the direction of the rear light's apparent position relative to the front light. Rear light to the left means turn left. This is counterintuitive to many sailors learning ranges for the first time — practice it in a harbor you know before relying on it in an unfamiliar approach.
Range lights are marked on charts with a magenta line indicating the bearing of the range. The bearing is given in the Light List. When approaching a range from seaward, bring the two lights into vertical alignment and maintain that alignment as you proceed up the channel. Ranges are especially useful in channels that curve, where simple compass headings become inadequate.
Not all ranges are lighted. Unlighted ranges use dayboards: a triangle or rectangle on a post, positioned so that alignment means you are on track. These are only usable in daylight and require good visibility to the range boards.
Special Marks: Yellow
Yellow marks serve special purposes not related to channel navigation. They indicate survey areas, fish havens, spoil areas, military exercise zones, or other special-purpose designations. A yellow buoy with a yellow "X" topmark is a special mark. They are not hazard markers — you can generally pass on either side — but they indicate that something atypical is happening in the area. The USCG Light List entry will specify the purpose. Fish haven markers are common in coastal waters of the Gulf of Mexico and indicate underwater artificial reef structures that are potential snag hazards.
The USCG Light List: The Full Reference
The USCG Light List, published annually and freely available from the USCG Navigation Center (navcen.uscg.gov), is the authoritative publication for all federally maintained aids. For each aid, it gives: official name and number, position (latitude and longitude), structure type, height of light above mean high water, nominal range (the distance at which the light is visible in clear weather), characteristic, and any special notes. The chart abbreviation is a shorthand version of the same data — "Fl R 4s 15ft 4M" tells you it is a flashing red light, four-second period, 15 feet above water, visible four miles. The Light List expands that into full detail including the structure description and any seasonal changes.
Before entering any unfamiliar harbor with lighted aids, cross-reference the chart symbols against the Light List. Charts are updated on a regular cycle but may not reflect recent changes to aid characteristics. The Light List is updated weekly online. If a light is not where the chart shows it, or flashes in a pattern you do not recognize, the Light List is the reference to check before concluding the light has moved.