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.

THE POINT, LINE, AND PLANE

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infinite in extent and in no way limited by their traces or by any
lines which locate them.
When a plane is oblique to G-L, as the plane S in Figs. 31 and 32,
it passes through all the quadrants. Its horizontal trace extends both
in front and back of G-L and its vertical trace extends both above
and below G-L. The traces S-s,, and S-s" limit that portion of the
plane in the first quadrant, the traces S-s, and S-s" limit that portion of the plane in the second quadrant, the traces S-s, and S-s'
limit that portion of the plane in the third quadrant, and the traces
S-s,, and S-s' limit that portion of the plane in the fourth quadrant.
FIG. 31 FIG. 32
40. To assume a Plane at Random. Assume the two traces at
random so long as they either intersect G-L at the same point or
are both parallel to G-L.
41. To assume a Series of Parallel Planes. Since the intersections of a series of parallel planes by an oblique plane is a series
of parallel straight lines, it follows that the horizontal traces of
parallel planes must be parallel; also that their vertical traces
must be parallel.
42. To assume a Straight Line in a Plane.
Principle. If a straight line in a plane pierces H it must pierce
it in the horizontal trace of the plane, and if it pierces V it must
pierce it in the vertical trace of the plane; for the horizontal trace
of the plane is the only line in common between the plane and H,
and the vertical trace of the plane is the only line in common
between the plane and V.