For suppose it is not, but, if possible, let it be in a more elevated plane as EGF, and let a plane be drawn through EGF;
it will then make, as section in the plane of reference, a straight line. [XI. 3]
Let it make it, as EF;
therefore the two straight lines EGF, EF will enclose an area: which is impossible.
Therefore the straight line joined from E to F is not in a plane more elevated;
therefore the straight line joined from E to F is in the plane through the parallel straight lines AB, CD.