When you see sidelights on another vessel, COLREGs show the vessels are crossing paths.

Cause and effect on the water can feel almost conversational. You see a glow on the horizon, a flash of green, a flash of red, and suddenly the entire scene clicks into place. If you’re studying the COLREGs, you know those sidelights aren’t just decorative; they’re telling you a story about how two boats will move relative to each other. So, what does it mean when you see the sidelights of another vessel? Here’s the practical, down-to-earth way to read that signal—and what it means for your own actions.

Sidelights aren’t random signals

Let’s start with the basics. Each vessel has red sidelight on the left (port) and green sidelight on the right (starboard), plus white stern and masthead lights when it’s dark. If you spot the other vessel’s sidelights—red and green visible together—that means you’re looking at a boat coming at you at an angle where the two boats are not moving in parallel lines. In everyday terms: you’re on a path that will cross with theirs, not a head-on approach or a neat, side-by-side glide.

If you’re picturing it, think of two cars on a winding road. When you catch sight of their headlights at an angle, you know you’re about to intersect lanes. The same logic applies on the water, just with colored lights doing the signaling.

Crossing paths, not reciprocal or parallel

So, seeing sidelights indicates a crossing situation. It’s not two boats gliding on parallel courses, and it’s not a head-on encounter. The rules for crossing require a particular kind of cooperation, even though one boat will end up adjusting course or speed to avoid a collision.

Here’s the essence: in a crossing scenario, the vessel that has the other on its starboard side should take steps to keep clear. The other vessel can maintain its course, but both parties need to be ready to adjust if the situation changes. It’s a dance—one that’s choreographed by the Rules of the Road so everyone can predict what the other will do.

Stand-on and give-way: who does what

You’ll hear the terms stand-on and give-way a lot in this context. If you see the sidelights of another vessel, you’re looking at a crossing where one boat acts as stand-on and the other as give-way. The stand-on vessel should keep its heading and speed unless other factors demand it. The give-way vessel—often the one with the other vessel on its starboard side—must act early and decisively to avoid a collision.

Why early action? Because waiting until the last second invites errors. Smooth, predictable maneuvers are safer for everyone on the water. A quick turn, a slight reduction in speed, or a minor course adjustment can be the difference between a near-mmiss and a safe passage.

A simple mental model to carry around

Let me explain with a mental picture you can reuse next time you’re at sea. When you see the other vessel’s sidelights, imagine you’re watching a crosswalk at dusk. The other boat is stepping into your lane from the side, not head-on. Your instinct should be to ask: “Who has the other on their starboard side?” If the answer is “I do,” you’re the stand-on vessel; keep your path. If the otherboat has you on its starboard side, they’re the stand-on and you need to give way.

This isn’t about playing a game of chicken. It’s about clear, early communication through motion. Even though you can’t hear a signal like “I’ll slow down,” you can see the intent in the changes you’re about to make.

A practical scenario, so it sticks

Imagine you’re closing in on a small vessel from the right front quarter. You can clearly see its sidelights as it angles toward you. You’re not approaching from directly ahead and you’re not offset enough to be mistaken for an overtaking situation. You’re in a crossing scenario.

What should you do? If you’re the stand-on boat, you maintain course and speed unless there’s a risk of collision. If there’s a real danger of collision, or if the other vessel’s path shifts into yours, you’re prepared to take early, substantial action—perhaps a gentle reduction in speed or a small course adjustment to pass clear behind the other boat. If you’re the give-way vessel, you act promptly to steer away and avoid risk, so the other vessel can continue safely.

A few common mix-ups to avoid

• Confusing sidelights with other signals: Seeing sidelights doesn’t automatically mean you’re in a stand-on situation. It signals crossing paths; you still have to assess which vessel is on the other’s starboard side.

• Waiting too long: Delaying action invites tighter margins and more stress on both boats. Early, predictable movements are safer.

• Overcorrecting: A big, sudden turn can be as dangerous as no turn at all. Small, deliberate adjustments usually work best.

How to apply this on real water

• Scan early: On approaching traffic, size up the other boat’s lights, track their bearing, and project whether you’re on a crossing path.

• Communicate with your actions: Your course and speed changes should be gentle and gradual so others can anticipate you.

• Stay clear in limited visibility: In fog or rain, the same rules apply, but the margin for error shrinks. Slow down and increase watchfulness, relying on radar and AIS if available.

• At night, stay predictable: Humans like to assume others will mirror their own behavior. Don’t rely on that assumption—be explicit in how you intend to pass.

A few quick memory anchors

• Sidelights visible = crossing paths.

• If the other vessel is on your starboard side, you (likely) have the stand-on role; keep course and speed unless needed.

• If you’re on the other vessel’s starboard beam, you’re the give-way vessel and should maneuver to avoid.

Why this matters beyond the test

Yes, the rules exist to pass tests and keep you legal on the water, but they’re rooted in safety. When two vessels cross, everyone deserves a clear plan. Sidelights give you a window into the other boat’s trajectory; they’re a cue to work with, not against. Mastery here isn’t about memorizing a single rule; it’s about developing a habit of looking for intent, communicating with your actions, and prioritizing a predictable, safe passage for everyone.

A little more nuance, if you’re curious

Rule-based navigation isn’t a one-size-fits-all recipe. Weather, currents, vessel type, and speed all tint the decision process. A sailboat under a jib may maneuver differently than a powered vessel with a planing hull. A fishing vessel in rough seas might make slow, deliberate adjustments. That’s why the Rules of the Road are written with flexibility, not rigidity: they guide you to a safe outcome even when conditions are less than ideal.

References you can trust (and why they matter)

You don’t need to memorize every line word-for-word to stay competent. The core ideas about sidelights signaling crossing paths, and the stand-on/give-way dynamic, are what you’ll rely on most. Practical navigation charts, radio communications etiquette, and ongoing training with reputable maritime authorities all reinforce these ideas at sea. If you ever question a scenario, a quick check against those practical guidelines will usually set you straight.

Bringing it together: a quick recap

• Seeing sidelights means you’re approaching on a crossing path.

• The stand-on vessel keeps its course and speed unless risk dictates otherwise.

• The give-way vessel acts early to avoid collision.

• In practice, aim for smooth, predictable actions and clear, quiet communication through your maneuvers.

• Stay mindful of conditions that can alter the plan, but don’t abandon the core rule: avoid a collision.

If you’re charting your way through the waters of COLREGs, remember this: lights are not just pretty signals at dusk. They’re a language. They tell you how two boats plan to move around each other. Read them, anticipate, and respond with calm, deliberate moves. That’s how you stay safe, confident, and respectful of everyone sharing the water.

Want more practical tidbits like this? Explore scenarios, diagrams, and real-world examples that bring the Rules of the Road to life. You’ll find that a little hands-on reasoning—paired with steady, practiced habits—goes a long way toward turning theory into confident, safe navigation.