COLREGs · Rule 21 · 33 CFR §83.21

Definitions (Lights)

Rule 21 defines each navigation light by color and arc: masthead light (white, 225° forward arc), sidelights (green 112.5° starboard, red 112.5° port), sternlight (white, 135° aft), towing light (yellow, 135° aft), all-round light (360°), and flashing light (≥120 flashes per minute).

What does Rule 21 actually say?#

Source

Verbatim 33 CFR §83.21, public domain. The international COLREGs text is published by the IMO and cannot be reproduced here; this U.S. wording tracks it closely.

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.

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 side lights 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.

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.

Towing light means a yellow light having the same characteristics as the “sternlight” defined in paragraph (c) of this Rule.

All-round light means a light showing an unbroken light over an arc of the horizon of 360 degrees.

Flashing light means a light flashing at regular intervals at a frequency of 120 flashes or more per minute.

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.

Practice this rule#

One real exam question on this rule, cited to source. No sign-up.

Course to steer to compensate for current

At what minimum frequency must a DR position be plotted when operating in pilot waters, according to standard DR practice?

Does Inland Rule 21 change anything?#

Inland Rule 21 is 33 CFR §83.21, reproduced above, and carries no exam-relevant divergence from COLREGs Rule 21. One answer serves both the International and the Inland version of a Rules of the Road question on this rule — which is worth knowing precisely because several neighbouring rules do not work that way.

21 of 38 rules differ somewhere; the five the exam leans on are Rule 9, Rule 10, Rule 24, Rule 28 and Rule 34, side by side on Inland vs International Rules.

Why it shows up on the exam#

Light-arc and light-color questions are among the most frequently tested items on every deck-license module. The single most common trap is confusing the 225° masthead arc with the 135° sternlight arc — together they ring the full 360°.

What does the exam actually test here?#

Sidelights
Green stbd · Red port, 112.5°
Each sidelight shows an unbroken arc from right ahead to 22.5° abaft the beam on its own side — green to starboard, red to port (33 CFR §83.21(b)).
Masthead vs sternlight
225° white fwd / 135° white aft
The masthead light covers 225° (right ahead to 22.5° abaft the beam each side); the sternlight covers the remaining 135° (67.5° from right aft on each side). 225° + 135° = a full 360° ring (§83.21(a),(c)).
Towing · all-round · flashing
Yellow 135° / 360° / ≥120 fpm
A towing light is yellow with the sternlight's 135° characteristics; an all-round light shows 360°; a flashing light flashes 120 or more times per minute (§83.21(d)–(f)).

Rule 21 — common exam questions#

What arc does a masthead light cover?

A masthead light is a white light showing an unbroken 225° arc — from right ahead to 22.5° abaft the beam on each side — fixed over the vessel's fore-and-aft centerline (33 CFR §83.21(a)).

What color are sidelights and what arc do they show?

Sidelights are a green light on the starboard side and a red light on the port side, each showing an unbroken 112.5° arc from right ahead to 22.5° abaft the beam on its own side (33 CFR §83.21(b)).

How many degrees is a sternlight, and how does it differ from an all-round light?

A sternlight is a white light covering a 135° arc, fixed 67.5° from right aft on each side (§83.21(c)). An all-round light shows an unbroken 360° arc (§83.21(e)). The 225° masthead arc plus the 135° sternlight arc together cover the full 360°.

What is a flashing light under Rule 21?

A flashing light is a light flashing at regular intervals at a frequency of 120 flashes or more per minute (33 CFR §83.21(f)). It differs from an all-round light, which burns continuously over 360°.

Authoritative source#

33 CFR §83.21 — this section with its neighbours. 33 CFR Part 83 is public domain; checked 2026-06-20.
COLREGs Rule 21 is published by the IMO and is authoritative internationally.

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