There are three glass tubes on the level ruler, all of which have bubbles inside. This is called a level bubble. The two sides of the level should be divided into anti-vibration ends. Place the level on the object to be measured. If the level bubble is in the middle of them, it means the plane is horizontal, so the level bubble in the middle is used to measure whether it is a horizontal plane. If the horizontal bubble is biased to one side, it means which side is high.