Elements of descriptive geometry, with applications to isometric projection and othe forms of one-plane projection; a text-book for colleges and ingineering schools by O. E. Randall.

4 DESCRIPTIVE GEOMETRY
12. Revolution of the Planes of Projection. The primary position of H is horizontal, and the primary position of V is vertical,
giving horizontal and vertical projections on two distinct planes
perpendicular to each other.
In order that projections upon H and V may be represented upon
a single plane, one of the planes of projection is revolved about
G-L as an axis until it is coincident with the other.
This last position of H and V is called the secondary position,
or position of coincidence.