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Rule 20: Application (Lights and Shapes) (COLREGS)

Last reviewed October 21, 2026

The short answer

Rule 20 says the lights and shapes rules apply in all weathers. Lights are shown from sunset to sunrise, and also by day in restricted visibility. Shapes are shown by day. Other lights may be shown only if they can't be mistaken for required lights, don't impair them, and don't interfere with the look-out.

The rule text

The Rules concerning lights shall be complied with from sunset to sunrise

International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 20

The Rules concerning shapes shall be complied with by day.

International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 20

What it means on the water

  • All weathers (20(a)).
  • Lights sunset to sunrise; no confusing or impairing lights (20(b)).
  • Lights also from sunrise to sunset in restricted visibility, and whenever deemed necessary (20(c)).
  • Shapes by day (20(d)). Technical specs are in Annex I (20(e)).

International vs Inland

Inland adds Rule 20(f): a vessel's navigation lights and shapes may be lowered if necessary to pass under a bridge.

Common exam traps

  • Thinking lights are only for night. Rule 20(c) requires them by day in restricted visibility.

Try 3 questions on Rule 20

  1. When are lights required?

    1. Only in fog
    2. Sunset to sunrise, and by day in restricted visibility
    3. Only at night in traffic
    4. Never by day
    Show answer

    B. Sunset to sunrise, and by day in restricted visibility Rule 20(b) and (c).

  2. When are shapes shown?

    1. By day
    2. At night
    3. Only in fog
    4. Only at anchor
    Show answer

    A. By day Rule 20(d).

  3. Inland only: when may lights be lowered?

    1. At anchor
    2. To pass under a bridge
    3. In fog
    4. Never
    Show answer

    B. To pass under a bridge Inland Rule 20(f).

Frequently asked questions

Do I need lights in daytime fog?

Yes. Rule 20(c) says required lights are also shown from sunrise to sunset in restricted visibility.

Can I show deck lights at night?

Only if they can't be mistaken for required lights, don't impair them, and don't interfere with the look-out (Rule 20(b)).

Inland difference?

Inland 20(f) allows lowering lights and shapes to pass under a bridge.

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