(a) Masthead light
A white light placed over the fore-and-aft centreline of the vessel, showing an unbroken light over an arc of 225°, from right ahead to 22.5° abaft the beam on either side.
COLREGs trainerPart C: Lights and Shapes
Rule 21
Definitions
Masthead, sidelights, sternlight, towing light, all-round, flashing.
Wording follows the International Regulations as printed in the US Coast Guard Navigation Rules handbook (2024), with British spelling, and is shortened in places. Download the rules
Read the whole of Rule 21 on one page
(a) Masthead light
A white light placed over the fore-and-aft centreline of the vessel, showing an unbroken light over an arc of 225°, from right ahead to 22.5° abaft the beam on either side.
(b) Sidelights
A green light on the starboard side and a red light on the port side, each showing an unbroken light over an arc of 112.5°, from right ahead to 22.5° abaft the beam on its side. In a vessel under 20 m they may be combined in one lantern on the centreline.
(c) Sternlight
A white light placed as nearly as practicable at the stern, showing an unbroken light over an arc of 135°: 67.5° from right aft on each side.
(d) Towing light
A yellow light having the same characteristics as the sternlight.
(e) All-round light
A light showing an unbroken light over an arc of the horizon of 360°.
(f) Flashing light
A light flashing at regular intervals at a frequency of 120 flashes or more per minute.
A training aid, not the official text. The COLREGs are an IMO convention, and your national authority publishes the version that applies in its waters. Check the rules themselves before you rely on them at sea. Download the rules.