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So, after the location of the column is determined, you need to make the base on the ceiling. To do this, you need to draw two parallel lines connecting two opposite walls. The distance between these lines should be equal to the width of the faces of the column. As a result, the ceiling should get a square (or a rectangle, depends on your idea), which will become the basis of the column. The main thing is that every two intersecting sides of this figure be strictly perpendicular to each other. With the help of plumb line, the same drawing is transferred to the floor.