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.

DESCRIPTIVE GEOMETRY
CHAPTER I
DEFINITIONS AND ASSUMPTIONS
1. The Subject defined. Descriptive geometry is that branch of
mathematics which seeks, through the medium of an exact process of graphic expression, to represent geometrical magnitudes
which occupy given positions in space, and also through the
same medium of expression to solve such problems as relate to
these magnitudes.
2. Representation of Magnitudes of Two Dimensions. A magnitude of two dimensions, such as a plane geometrical figure, may be
easily and directly represented, graphically, upon a single plane,
since every characteristic of such a magnitude may be determined
from a single standpoint of observation, and the whole may be
outlined upon the very plane in which the magnitude exists.
The diagrams connected with the statement and solution of
problems in plane geometry furnish an illustration of this fact.
3. Representation of Magnitudes of Three Dimensions. A magnitude of three dimensions does not exist in a single plane, neither
can its characteristics be completely determined from a single
standpoint of observation; therefore the process of representation
must necessarily be different from that employed in connection
with magnitudes of two dimensions.
4. Projection. Since the points and lines of magnitudes of three
dimensions do not exist in a single plane, as is the case with
magnitudes of two dimensions, it will be necessary to determine
some plane of representation and to establish some process by
which reference to this plane may be made.
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