COLREGs trainerPart C: Lights and Shapes
Rule 20
Lights from sunset to sunrise and in restricted visibility; shapes by day; in all weathers.
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
(a) Rules in this Part shall be complied with in all weathers. (b) The Rules concerning lights shall be complied with from sunset to sunrise, and during such times no other lights shall be exhibited, except such lights as cannot be mistaken for the lights specified in these Rules or do not impair their visibility or distinctive character, or interfere with the keeping of a proper look-out.
The lights prescribed by these Rules shall, if carried, also be exhibited from sunrise to sunset in restricted visibility and may be exhibited in all other circumstances when it is deemed necessary.
(d) The Rules concerning shapes shall be complied with by day. (e) The lights and shapes specified in these Rules shall comply with the provisions of Annex I to these Regulations. In plain words: navigation lights go on at sunset and stay on until sunrise. In restricted visibility (fog, heavy rain) they go on by day too, if you carry them. Shapes are for daylight. Any other light you show at night must not be mistaken for a navigation light, hide one, or spoil the look-out.
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.