水平面

The plumbline is always perpendicular to the horizontal plane.

铅垂线总是垂直于水平面的。

ROLL is the instantaneous value of the angle between the reference plane and the horizontal, measured in an athwartship vertical plane.

横摇以从左到右通过舰身的垂直面，量度参考面与水平面之所形成的即时角度。