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Rule 21: Definitions (Lights)

1The rule in plain words

In one sentence…

COLREG defines each light by its colour, position and visible sector on the horizon.

Masthead-light identification detail, not a scaled sector diagram. This white light covers 225°, from right ahead to 22.5° abaft the beam on each side. It is placed over the vessel’s fore-and-aft centreline.

What is the masthead light?

It is white, sits over the fore and aft centerline, and shows an unbroken light over a 225° horizon arc: from right ahead to 22.5° abaft the beam on either side.

What are sidelights?

The starboard light is green and the port light red; each shows an unbroken light over a 112.5° horizon arc from right ahead to 22.5° abaft the beam on its respective side.

What are the sternlight and towing light?

The sternlight is white, placed as nearly as practicable at the stern, and covers an unbroken 135° (67.5° from right aft on each side); the towing light is yellow and has the same characteristics.

What do all-round and flashing mean?

The first shows an unbroken light over a 360° horizon arc; the second flashes at regular intervals, at a frequency of 120 or more per minute.

Official text

IMO COLREG 1972U.S. Coast Guard — International Navigation Rules (2024)

Full wording of the rule, reproduced from the official source below.

(a) Masthead light means a white light placed over the fore and aft centerline of the vessel showing an unbroken light over an arc of the horizon of 225° and so fixed as to show the light from right ahead to 22.5° abaft the beam on either side of the vessel. (b) Sidelights means a green light on the starboard side and a red light on the port side each showing an unbroken light over an arc of the horizon of 112.5° and so fixed as to show the light from right ahead to 22.5° abaft the beam on its respective side. In a vessel of less than 20 m in length the sidelights may be combined in one lantern carried on the fore and aft centerline of the vessel. (c) Sternlight means a white light placed as nearly as practicable at the stern showing an unbroken light over an arc of the horizon of 135° and so fixed as to show the light 67.5° from right aft on each side of the vessel. (d) Towing light means a yellow light having the same characteristics as the sternlight defined in paragraph (c) of this Rule. (e) All-round light means a light showing an unbroken light over an arc of the horizon of 360°. (f) Flashing light means a light flashing at regular intervals at a frequency of 120 flashes or more per minute.
Source:
Sidelight identification detail, not a scaled sector diagram. Red marks port and green starboard. Each light covers 112.5°, from right ahead to 22.5° abaft the beam on its own side.
Sternlight identification detail, not a scaled sector diagram. This white light covers 135° centred on right astern: 67.5° to each side. It is placed as nearly as practicable at the stern.
Towing-light identification detail, not a scaled sector diagram. This yellow light has the same 135° stern sector as a sternlight: 67.5° on each side of right astern. It is distinct from the white sternlight.

2Key points

  1. 1.

    Masthead: white over the centreline · 225° towards ahead and both sides.

  2. 2.

    Sidelights: green starboard · red port · 112.5° each.

  3. 3.

    Sternlight 135° · towing light yellow with the same sector.

  4. 4.

    All-round 360° · flashing: 120 or more per minute.

3Frequent mistakes

  • Wrong: "The masthead light is any white light facing ahead."

    Right: It must be over the fore and aft centerline and cover 225° from right ahead to 22.5° abaft the beam.

  • Wrong: "The towing light is red, like the port sidelight."

    Right: It is yellow and has the same characteristics as the sternlight.

  • Wrong: "A flashing light may flash at any rate."

    Right: It flashes at regular intervals: 120 or more flashes per minute.

4Exam keys & interpretation

Exam keys

  • "Over the centreline" identifies masthead; "as nearly as practicable at the stern" identifies sternlight.

  • "Same characteristics" does not mean the same colour: the towing light is yellow.

  • "120 or more per minute" is a frequency, not a horizon arc.

How to read it

  • Read each definition as a label: name, colour, position and visible sector; do not mix an arc with a frequency.

  • If a question changes "abaft the beam" to "from right aft", check which light and measurement it describes.

5Test your knowledge

Question 1/4

Which description matches the masthead light?

More questions on this rule →
Free practice test: 40 questions with answersTake it as a timed test, or read every answer with its explanation. No sign-up.

Keep studying…

22Next rule →Visibility of Lights
Previous: Rule 20 · Application (Lights)

Reviewed by Ramon Curto, Nautical teacher at IECNPC · Last reviewed:

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Frequently asked questions about COLREG Rule 21

What arc does a masthead light cover under COLREG Rule 21?

225 degrees. The masthead light is a white light shown from dead ahead to 22.5 degrees abaft the beam on each side (Rule 21(a)).

What are the arcs of the sidelights and the sternlight?

Each sidelight shows over 112.5 degrees, from dead ahead to 22.5 degrees abaft the beam — green to starboard, red to port. The sternlight shows over 135 degrees, from dead astern to 67.5 degrees on each side.

What colour is a towing light, and how fast is a flashing light?

A towing light is yellow with the same characteristics as a sternlight (Rule 21(d)). A flashing light flashes at regular intervals at a frequency of 120 flashes or more per minute (Rule 21(f)).