COLREGs Rule 21: Definitions of navigation light types
Last reviewed: October 14, 2026
Rule 21 defines the lights that other rules refer to, including their color and arc of the horizon. The Inland rule adds a definition of special flashing light and adds placement wording for vessels of less than 12 meters in length.
What it says
A masthead light is a white light over the fore and aft centerline that shows an unbroken light over an arc of the horizon of 225°, from right ahead to 22.5° abaft the beam on either side.
COLREGs Rule 21(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°”
International: 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.
COLREGs Rule 21(b)“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.”
Inland: On a vessel of less than 12 meters in length, the masthead light must be placed as nearly as practicable to the fore and aft centerline of the vessel.
COLREGs Rule 21(a)“except that on a vessel of less than 12 meters in length the masthead light shall be placed as nearly as practicable to the fore and aft centerline of the vessel.”
A sternlight is a white light placed as nearly as practicable at the stern, showing an unbroken light over an arc of the horizon of 135°, 67.5° from right aft on each side of the vessel.
COLREGs Rule 21(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°”
A towing light is a yellow light with the same characteristics as the sternlight defined in paragraph (c) of this Rule.
COLREGs Rule 21(d)“Towing light means a yellow light having the same characteristics as the sternlight defined in paragraph (c) of this Rule.”
A flashing light flashes at regular intervals at a frequency of 120 flashes or more per minute.
COLREGs Rule 21(f)“Flashing light means a light flashing at regular intervals at a frequency of 120 flashes or more per minute.”
Inland: A special flashing light is a yellow light flashing at regular intervals at a frequency of 50 to 70 flashes per minute, placed as far forward and as nearly as practicable on the fore and aft centerline of the tow.
COLREGs Rule 21(g)“Special flashing light means a yellow light flashing at regular intervals at a frequency of 50 to 70 flashes per minute”
Common questions
What is an all-round light?
An all-round light is a light showing an unbroken light over an arc of the horizon of 360°.
“All-round light means a light showing an unbroken light over an arc of the horizon of 360°.”
Can sidelights be combined in one lantern?
International: yes, in a vessel of less than 20 m in length, on the fore and aft centerline. Inland: yes, on a vessel of less than 20 meters in length, and if the vessel is under 12 meters the lantern must be placed as nearly as practicable to the centerline.
“On a vessel of less than 20 meters in length the sidelights may be combined in one lantern carried on the fore and aft centerline of the”
What color is a towing light?
A towing light is yellow and has the same characteristics as the sternlight defined in paragraph (c) of this Rule.
“Towing light means a yellow light having the same characteristics as the sternlight defined in paragraph (c) of this Rule.”
The text of COLREGs Rule 21
U.S. Coast Guard, Navigation Rules and Regulations Handbook
Rule 21: Definitions
International (COLREGs)
(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.
Inland Rules
Definitions
(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 degrees and so fixed as to show the light from right ahead to 22.5 degrees abaft the beam on either side of the vessel, except that on a vessel of less than 12 meters in length the masthead light shall be placed as nearly as practicable to the fore and aft centerline of the vessel.
(b) Sidelights mean 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 degrees and so fixed as to show the light from right ahead to 22.5 degrees abaft the beam on its respective side. On a vessel of less than 20 meters in length the sidelights may be combined in one lantern carried on the fore and aft centerline of the vessel, except that on a vessel of less than 12 meters in length the sidelights when combined in one lantern shall be placed as nearly as practicable to 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 degrees and so fixed as to show the light 67.5 degrees 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 degrees.
(f) Flashing light means a light flashing at regular intervals at a frequency of 120 flashes or more per minute.
(g) Special flashing light means a yellow light flashing at regular intervals at a frequency of 50 to 70 flashes per minute, placed as far forward and as nearly as practicable on the fore and aft centerline of the tow and showing an unbroken light over an arc of the horizon of not less than 180 degrees nor more than 225 degrees and so fixed as to show the light from right ahead to abeam and no more than 22.5 degrees abaft the beam on either side of the vessel.
The plain-English explanation on this page was written with AI from the regulation text above and checked automatically before publishing: every quote appears in the text, every number comes from it, and a second model confirmed each statement adds nothing the text doesn't say. Regulation text current as of August 12, 2024. The regulation text governs. RegKnot is not affiliated with the U.S. Coast Guard.