To set up a straight line at right angles to a given plane from a given point in it.

Let the plane of reference be the given plane and *A* the point in it.

It is required to set up from the point *A* a straight line at right angles to the plane of reference.

From an elevated point *B* draw *BC* perpendicular to the plane of reference, and draw *AD* parallel to to *BC* through the point *A.*

Then since *AD* and *BC* are two parallel straight lines, and one of them, *BC,* is at right angles to the plane of reference, therefore the remaining one, *AD,* is also at right angles to the plane of reference.

Therefore *AD* is set up at right angles to the given plane from the point *A* in it.

Q.E.F.