Page 6.2.

This is still not an easy view to draw, so what we do is fold the planes flat, along the XY and the YY lines. By doing this we can draw it on a sheet much more easily. Here you see the planes been folded back onto a lined out sheet. The V.P. remains where it is, because we assume that the sheet is vertical. We can also see here why the line of intersection between the E.V.P. and the H.P. is part of the XY also.