Verticality detection of marble ruler

When checking the perpendicularity of the two working faces of the marble square ruler, first place the ruler on the marble flat plate, and let the gauge seat equipped with a dial indicator with a division value of 0.001mm pass through the standard round rod and zero on the standard marble square box. Then, in the same way, place the dial gauge holder tightly against the other working face of the marble square, and the reading of the dial gauge is the perpendicularity error of that side. Take the error value for the verticality of the marble ruler.