The geometry applied to the DIY is sometimes problematic: here is a simple explanation to draw a right angle to the wall and finally use the theorem of Pythagoras in real life. Convenient!
- Mark a point P on the wall and draw a segment multiplied by three to get a point A to post on the wall.
- From P, draw an arc with the same segment multiplied by four (to apply the Pythagorean theorem to the right triangle).
- Since A, draw another circle in five segments.
- The two arcs intersect in B.
- The line PB is perpendicular to the wall.