Draw a perpendicular line.

A line perpendicular to a given straight line can be drawn in two ways. The first is to draw a perpendicular from a specific point lying on the line p, second – from the point outside the straight line p. We will use a compass and a ruler to draw both constructions.

In the first case, we choose any point A lying on the line p and draw an arc from it until it intersects with the straight line at points B and C (figure a). From the points B and C received, radius greater than AB, we circle the arcs, that intersect at point D. Then we connect point D with point A and we get the desired perpendicular to the line p. The result can be checked with a triangle or protractor.

In the second case, drawing a perpendicular from point A, lying outside the line p, it's about that, that from point A we draw an arc with such a radius, so that it intersects the line p at two points B and C (drawing b). From these points we draw arcs with the same radii, that intersect at point D. Combining points A and D we get perpendicular to the straight line p.