Euclid's Elements Book I

1. Let A be the given point, and BC the given straight line. It is required to draw through the point A a straight line parallel to BC. (Every point can be dragged.)
2. Let a point D be taken at random on BC (you can drag it), and let AD be joined.

   Postulate 1

   Postulate 1: To draw a straight line from any point to any point.
3. On the straight line DA, at the point A on it, let the angle DAE be constructed equal to the angle ADC.

   Proposition I.23

   Proposition I.23: On a given straight line and at a point on it to construct a rectilineal angle equal to a given rectilineal angle.
4. Let the straight line EA be produced in a straight line to F.

   Postulate 2

   Postulate 2: To produce a finite straight line continuously in a straight line.
5. Since the straight line AD falling on the two straight lines BC, EF makes the alternate angles EAD, ADC equal, EF is parallel to BC. Therefore through the given point A the straight line EAF has been drawn parallel to BC.

   Proposition I.27

   Proposition I.27: If a straight line falling on two straight lines make the alternate angles equal to one another, the straight lines will be parallel to one another.

Being what it was required to do.