Now, since the plane AB is at right angles to the plane of reference, and DE has been drawn in the plane AB at right angles to AD, their common section,
therefore DE is at right angles to the plane of reference. [XI. Def. 4]
Similarly we can prove that DF is also at right angles to the plane of reference.
Therefore from the same point D two straight lines have been set up at right angles to the plane of reference on the same side: which is impossible. [XI. 13]
Therefore no straight line except the common section DB of the planes AB, BC can be set up from the point D at right angles to the plane of reference.