Pantometria — A Geometrical Treatize of Measuring

...place one foot of your compass in E, draw an arc directly under C, and then, fixing one foot of your compass in D, cross the former arc at point F. Finally, draw the straight line CF, for that is a perpendicular to the line AB.

A geometric diagram showing the construction of a perpendicular line. A horizontal line segment is labeled A, D, E, B. Above it is point C. Two arcs from points D and E intersect below the line at point F. A vertical line segment connects C and F, passing through the horizontal line. The word "Perpendiculer" is written vertically along this segment.

Pythagoras' invention might here take place, who did find these numbers 3, 4, and 5, or like measures, to make a right angle.
A right-angled triangle with sides labeled 3, 4, and 5, demonstrating the Pythagorean theorem.

The Third Chapter.

From any point assigned, to extend a parallel to any other straight line lying in the surface.

From the assigned point, let fall a perpendicular to that line. From some other point on that line, create a perpendicular, as you were taught in the last chapters. Then, opening your compass to the length of the perpendicular let fall from the assigned point, measure out that same length on the perpendicular you just created, beginning from the base line. Then, laying a ruler to the assigned point and the end of that length, draw a straight line; that shall be a parallel to the other.