What lights must a vessel not under command display at night?

A vessel not under command is defined as one that is unable to maneuver due to some disabling condition. To be visible to other vessels at night, such a vessel must display two red lights vertically. The red lights signify that it is not capable of taking action to avoid collisions and alerts other vessels to exercise caution when navigating near it.

The signal of two red lights helps communicate the vessel's status clearly, distinguishing it from other types of vessels, such as those that are anchored or under sail. While other light combinations are important for different types of vessels, they do not represent the specific situation of being "not under command." Therefore, displaying two red lights is a critical safety measure to prevent maritime accidents and ensure safe navigation.