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Rule 21: Definitions (Lights) (COLREGS)

Last reviewed October 21, 2026

The short answer

Rule 21 defines the lights. A masthead light is white over 225 degrees, from right ahead to 22.5 degrees abaft the beam each side. Sidelights are green (starboard) and red (port), 112.5 degrees each. A sternlight is white over 135 degrees. A towing light is yellow like a sternlight. A flashing light flashes 120 or more times a minute.

The rule text

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.

International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 21

Flashing light means a light flashing at regular intervals at a frequency of 120 flashes or more per minute.

International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 21

What it means on the water

  • Masthead: white, 225 degrees (21(a)).
  • Sidelights: green starboard, red port, 112.5 degrees each; may be combined in one lantern under 20 meters (21(b)).
  • Sternlight: white, 135 degrees, 67.5 degrees from right aft each side (21(c)).
  • Towing light: yellow, same as sternlight (21(d)). All-round: 360 degrees (21(e)).

International vs Inland

Inland 21(g) adds the special flashing light: yellow, 50 to 70 flashes per minute, arc of not less than 180 nor more than 225 degrees. Inland 21(a) and (b) also add centerline placement wording for vessels under 12 meters.

Common exam traps

  • Giving the sternlight 112.5 degrees. It's 135.
  • Calling the towing light white. It's yellow.

Try 3 questions on Rule 21

  1. Arc of a masthead light?

    1. 112.5 degrees
    2. 135 degrees
    3. 225 degrees
    4. 360 degrees
    Show answer

    C. 225 degrees Rule 21(a).

  2. Color of a towing light?

    1. White
    2. Red
    3. Yellow
    4. Green
    Show answer

    C. Yellow Rule 21(d).

  3. A flashing light flashes at least:

    1. 50 per minute
    2. 70 per minute
    3. 120 per minute
    4. 30 per minute
    Show answer

    C. 120 per minute Rule 21(f).

Frequently asked questions

What is the arc of a sidelight?

112.5 degrees, from right ahead to 22.5 degrees abaft the beam on its side (Rule 21(b)).

What is a special flashing light?

An Inland-only yellow light flashing 50 to 70 times per minute (Inland 21(g)).

What is the sternlight arc?

135 degrees (Rule 21(c)).

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