Although rulers are often used as straightedges, the graduation marks may not be used. No measurements are allowed. The straightedge is used only for drawing lines. Given any two distinct points, this instrument can draw the set of all points that are collinear with them. Rulers and protractors have their place in geometry, but these are not construction instruments.
All measurements are approximations.
In reality, the compass and straightedge also are subject to error. A line has zero width, but a drawing of a line does not. However, construction is a theoretical exercise. If we were not limited by the physical imperfections of the instruments, the drawing surface, and the person using them, then construction instruments would render exact results. This is not true of measurement instruments.