Rules 20–31
Lights & shapes — overview
Lights & shapes
Rules 20–31 prescribe the lights shown from sunset to sunrise and the day shapes — covered vessel by vessel in the Lights topic.
In plain language
Part C tells every vessel what lights to show from sunset to sunrise (and in restricted visibility by day), and what shapes to show by day. The pattern of lights encodes what a vessel is, what she is doing, her aspect, and roughly her size — which is exactly what you need to apply Rules 12–18 at night. boatrules.com covers every configuration in rotatable 3D on the Lights & Shapes pages.
Takeaway
Lights tell you what a vessel is, what she's doing, and which way she's heading — learn them in 3D on the Lights pages.
This rule differs under the US Inland Rules. Change the rule set in language and regional settings to see the difference.