What is indicated if a vessel sees the sidelights of another vessel?

When a vessel sees the sidelights of another vessel, it indicates that the two vessels are crossing paths. This visibility of the sidelights means the observer can see the red and green lights, which represent the side lights of the two vessels facing each other. According to the COLREGs, when two vessels are on a collision course and one sees the sidelights of the other, it indicates they are approaching at an angle, typically requiring one of the vessels to give way.

The scenario is not indicative of a reciprocal course, as that would mean both vessels are facing each other bow to bow, where visibility of both sidelights would not occur. Additionally, if a vessel were overtaking another, only the stern light of the overtaken vessel would be visible, not the sidelights. Therefore, in the situation where sidelights are visible, it thereby confirms that the vessels are indeed crossing paths and requires the appropriate action to avoid a collision.