The main idea of vertical ruler

The use of the ruler is very simple and convenient. Suppose a house sitting at noon (sitting north to south), just take out the floor plan of the house, then put the vertical ruler on the floor plan to find out the orientation of sitting (sitting north) in the house, and finally point the sub-square (north) palace on the vertical ruler to the sub-square (north) palace of the house, so that all the orientations in the house can be seen at a glance.