Proposition 5.2: At the intersection of two distinct primary baselines there is a secondary baseline which can not coincide with either of these primary baselines.
Pf: Let and be two planes which intersect in distinct primary baselines. Let P be the point of intersection of these baselines. Now, the intersection of and is a line l which does not lie in (otherwise the baselines would not be distinct). The projection of l from V into , is a secondary baseline which passes through P. Suppose, without loss of generality, that this secondary baseline coincided with the primary baseline of . The plane containing the primary baseline and l is clearly , but because l projects to the baseline, V must also be in this plane. This contradiction shows that the secondary baseline must be distinct from both primary baselines.