What does two vertically disposed red lights without other lights indicate?

Two vertically disposed red lights without other lights on a vessel indicate that the vessel is not making way and is not under command. This configuration serves as an important signal to other vessels, providing a clear visual indication of the vessel's status. A vessel not under command is one that cannot maneuver as required, typically due to some incapacitating issue, and therefore needs to communicate this to prevent collisions.

This signaling plays a critical role in maritime safety, as it alerts nearby vessels to exercise caution and avoid close proximity to the vessel that is not able to maneuver properly. The absence of additional lights reinforces the state of being not under command, as there are no other signals present that might imply a different circumstance. Understanding the specific combinations of lights and their meanings is essential for safe navigation and adherence to the COLREGs.