1The rule in plain words
In one sentence…
When you see red-white-red, think "restricted ahead": the vessel is restricted in her ability to manoeuvre.
What does a vessel not under command show?
Two all-round red lights in a vertical line and two balls or similar shapes. When making way, add sidelights and a sternlight.
How do you recognise restricted ability to manoeuvre?
Three all-round lights: red, white, red; and three shapes: ball, diamond, ball. When making way, add a masthead light or lights, sidelights and a sternlight; at anchor, add the signals in Rule 30.
What do dredging or diving signals show when an obstruction exists?
Two red lights or balls show the obstruction; two green lights or diamonds show the passing side. If diving size makes the full display impracticable: three red-white-red lights and a rigid replica of flag "A", at least 1 metre high, visible all round.
What do mine-clearance vessels show?
In addition to the appropriate Rule 23 or 30 signals, they show three all-round green lights or three balls: it is dangerous to approach within 1000 meters. Vessels less than 12 metres are exempt, except diving vessels.
2See it in 3D
Turn the vessel or pick bow, stern, port or starboard: this is how her lights look at night.
An interactive 3D illustration is shown here. The same content is described in the rule text and key takeaways below.
3Key points
- 1.
Not under command → two red · two balls · making way, sidelights + sternlight.
- 2.
Restricted ability → red-white-red · ball-diamond-ball · at anchor, Rule 30.
- 3.
Obstruction = red · passing side = green; impracticable diving display → rigid flag A.
- 4.
Mine clearance → green-green-green · danger within 1000 m.
4Frequent mistakes
Wrong: "Red-white-red means not under command."
Right: No: it marks restricted ability to manoeuvre; not under command uses two red lights.
Wrong: "In dredging, red means the passing side and green the obstruction."
Right: Red marks the obstruction; green marks the side where another vessel may pass.
Wrong: "Rule 27 signals are distress signals."
Right: No: distress signals are in Annex IV to these Regulations.
5Exam keys & interpretation
Exam keys
When the question says "making way through the water", add sidelights and sternlight; do not treat it as mere underway status.
For a severely restricting tow, add Rule 27(b)(i)-(ii) lights or shapes to Rule 24(a).
"Less than 12 metres" excludes exactly 12 metres and does not exempt a vessel engaged in diving.
How to read it
A tow that severely restricts the towing vessel and her tow combines towing signals with restricted-ability lights and shapes.
These Rule 27 signals do not request assistance; for vessels in distress, consult Annex IV to these Regulations.
6Official text
IMO COLREG 1972U.S. Coast Guard — International Navigation Rules (2024)
Full wording of the rule, reproduced from the official source below.
(a) A vessel not under command shall exhibit:
(i) two all-round red lights in a vertical line where they can best be seen;
(ii) two balls or similar shapes in a vertical line where they can best be seen;
(iii) when making way through the water, in addition to the lights prescribed in this paragraph, sidelights and a sternlight.
(b) A vessel restricted in her ability to maneuver, except a vessel engaged in mine clearance operations, shall exhibit:
(i) three all-round lights in a vertical line where they can best be seen. The highest and lowest of these lights shall be red and the middle light shall be white;
(ii) three shapes in a vertical line where they can best be seen. The highest and lowest of these shapes shall be balls and the middle one a diamond;
(iii) when making way through the water, a masthead light or lights, sidelights and a sternlight, in addition to the lights prescribed in subparagraph (i);
(iv) when at anchor, in addition to the lights or shapes prescribed in subparagraphs (i) and (ii), the light, lights or shape prescribed in Rule 30.
(c) A power-driven vessel engaged in a towing operation such as severely restricts the towing vessel and her tow in their ability to deviate from their course shall, in addition to the lights or shapes prescribed in Rule 24(a), exhibit the lights or shapes prescribed in subparagraphs (b)(i) and (ii) of this Rule.
(d) A vessel engaged in dredging or underwater operations, when restricted in her ability to maneuver, shall exhibit the lights and shapes prescribed in subparagraphs (b)(i), (ii) and (iii) of this Rule and shall in addition, when an obstruction exists, exhibit:
(i) two all-round red lights or two balls in a vertical line to indicate the side on which the obstruction exists;
(ii) two all-round green lights or two diamonds in a vertical line to indicate the side on which another vessel may pass;
(iii) when at anchor, the lights or shapes prescribed in this paragraph instead of the lights or shape prescribed in Rule 30.
(e) Whenever the size of a vessel engaged in diving operations makes it impracticable to exhibit all lights and shapes prescribed in paragraph (d) of this Rule, the following shall be exhibited:
(i) three all-round lights in a vertical line where they can best be seen. The highest and lowest of these lights shall be red and the middle light shall be white;
(ii) a rigid replica of the International Code flag “A” not less than 1 meter in height. Measures shall be taken to ensure its all-round visibility.
(f) A vessel engaged in mine clearance operations shall, in addition to the lights prescribed for a power-driven vessel in Rule 23 or to the lights or shape prescribed for a vessel at anchor in Rule 30 as appropriate, exhibit three all-round green lights or three balls. One of these lights or shapes shall be exhibited near the foremast head and one at each end of the fore yard. These lights or shapes indicate that it is dangerous for another vessel to approach within 1000 meters of the mine clearance vessel.
(g) Vessels of less than 12 meters in length, except those engaged in diving operations, shall not be required to exhibit the lights and shapes prescribed in this Rule.
(h) The signals prescribed in this Rule are not signals of vessels in distress and requiring assistance. Such signals are contained in Annex IV to these Regulations.7Test your knowledge
What must a vessel not under command exhibit when making way through the water?
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