Paragraph (a): Sailing vessel underway
A sailing vessel underway shall exhibit:
(i) sidelights;
(ii) a sternlight.
COLREGs trainerPart C: Lights and Shapes
Rule 25
Sailing Vessels and Vessels Under Oars
Sidelights and sternlight; optional red-over-green at masthead.
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 25 on one page
Paragraph (a): Sailing vessel underway
A sailing vessel underway shall exhibit: (i) sidelights; (ii) a sternlight.
Paragraph (b): Combined lantern (< 20 m)
In a sailing vessel of less than 20 m in length the lights prescribed in paragraph (a) may be combined in one lantern carried at or near the top of the mast where it can best be seen.
Paragraph (c): Optional red-over-green
A sailing vessel underway may, in addition to the paragraph (a) lights, exhibit at or near the top of the mast two all-round lights in a vertical line: the upper red and the lower green. These shall not be shown together with the combined lantern of paragraph (b).
Paragraph (d): Small craft and vessels under oars
(i) A sailing vessel of less than 7 m shall, if practicable, exhibit the paragraph (a) or (b) lights; if not, she shall have ready at hand an electric torch or lighted lantern showing a white light, shown in time to prevent collision. (ii) A vessel under oars may exhibit those lights, but if not, shall likewise have a white light ready to show in time to prevent collision.
Paragraph (e): Sail and machinery together
A vessel proceeding under sail when also being propelled by machinery shall exhibit forward, where it can best be seen, a conical shape with the apex downwards.
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.