While underway at night, you see two yellow lights displayed in a vertical line. This should indicate to you a(n) __________.

When you see two yellow lights displayed in a vertical line while underway at night, it indicates a vessel that is pushing ahead. According to the navigation rules, the specific arrangement of lights is crucial for understanding the status and operation of vessels after dark.

In this case, the two yellow lights signal that the vessel is engaged in a pushing operation, meaning it is pushing against another vessel or a barge ahead. This visual cue allows other mariners to recognize the vessel's role and navigate appropriately, ensuring safety and proper communication on the water.

Other configurations of lights represent different scenarios, such as a vessel at anchor or a vessel being towed, which helps distinguish their operational status. It's important to be familiar with these light signals to ensure safe navigation, especially at night when visibility is reduced.