Two power-driven vessels are crossing. Who gives way and how?
The one that has the other on her own starboard side. She turns to starboard and passes astern: crossing ahead is not allowed.
Rule 15.
Part of the convention When you can see each other
Two power-driven vessels crossing: the one that has the other on her own starboard side keeps out of the way and, if the circumstances of the case admit, avoids crossing ahead of the other, that is, turns to starboard and passes astern.
At sea it looks like this
At night you see a red sidelight on your starboard side: you are looking at her port side, she is crossing from right to left ahead of you. Red means stop, you give way. A green light means you see her starboard side, she gives way, and you keep your course and speed.
The rule says«…the vessel which has the other on her own starboard side shall keep out of the way and shall, if the circumstances of the case admit, avoid crossing ahead of the other vessel.»
Check yourself
At night you see only a red sidelight on your starboard side. What do you do?
Red means you see her port side, she is crossing right to left ahead of you. You give way: reduce speed and/or turn to starboard.
Rule 15. Mnemonic: red means stop, green means go.
At night you see only a green sidelight on your starboard side. What do you do?
You see her starboard side, so she gives way. Keep your course and speed, but watch her: if she does nothing, act yourself.
Rule 15 and 17.