There are three glass tubes on the horizontal ruler. There are air bubbles in its glass tube. This is called a level bubble. On both sides of the horizontal ruler are divided into shock -proof end. Put the horizontal ruler on items that need to be measured. If the level is soaked in the middle of them, it means that the plane is a horizontal state, so the middle level bubble is to measure whether the horizontal plane is measured.
If the horizontal baunt is biased, it means which side is high.