Rule 21 defines the types of navigation lights:
Masthead light
Sidelights
Sternlight
Towing light
All-round light
IMO COLREG 1972Official text
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.Recognition Sequence
Classify the vessel state first: underway, making way, stopped, at anchor, aground, towing, fishing, pilotage or special condition.
Read special lights vertically from top to bottom before using sidelights and sternlight to confirm aspect.
Use the complete display and any vessel length stated in the scenario.
Optional second masthead lights, two anchor lights or deck illumination do not by themselves prove a minimum vessel length.
Exam Focus
Avoid identifying a vessel from one colour alone.
Many mistakes come from spotting a red light and guessing before checking the full pattern.
If the question mentions 'making way', 'underway but stopped', 'at anchor' or 'aground', that wording usually determines which extra lights or shapes appear.
Key Takeaways
Masthead light covers 225 degrees (ahead to 22.5 abaft beam)
Sidelights cover 112.5 degrees each
Sternlight covers 135 degrees
All-round light covers full 360 degrees
Common Mistakes
Confusing the arcs of visibility for different light types
Forgetting towing light is yellow, same arc as sternlight
Test Your Knowledge
Practise this topic and review the explanation for each answer.
More rules in this Part
- Power-driven Vessels UnderwayPower-driven vessels underway shall exhibit masthead light(s), sidelights, and sternlight. Vessels 50m+ require two masthead lights.
- Towing and PushingVessels towing or pushing display additional masthead lights and a yellow towing light. Three masthead lights if tow exceeds 200m.
- Sailing Vessels Underway and Vessels Under OarsSailing vessels show sidelights and sternlight. May optionally show red over green all-round lights at the masthead.
- Fishing VesselsFishing vessels show green over white when trawling, red over white for other fishing, and sidelights plus sternlight when making way.
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