1The rule in plain words
In one sentence…
COLREG defines each light by its colour, position and visible sector on the horizon.
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.
2Key points
- 1.
Masthead: white over the centreline · 225° towards ahead and both sides.
- 2.
Sidelights: green starboard · red port · 112.5° each.
- 3.
Sternlight 135° · towing light yellow with the same sector.
- 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.
5Official 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.6Test your knowledge
Which description matches the masthead light?
Keep studying…
Last reviewed: