Problem VII
To Draw Any Rectilinear Quadrilateral Figure, Given in Position and Magnitude, in a Horizontal Plane
Let A B C D (Fig. 19.) be the given figure.
Join any two of its opposite angles by the line B C.
Draw first the triangle A B C. (Problem VI.)
And then, from the base B C, the two lines B D, C D, to their vanishing-points, which will complete the figure. It is unnecessary to give a diagram of the construction, which is merely that of Fig. 18. duplicated; another triangle being drawn on the line A C or B C.
Corollary
It is evident that by this application of Problem VI. any given rectilinear figure whatever in a horizontal plane may be drawn, since any such figure may be divided into a number of triangles, and the triangles then drawn in succession.
More convenient methods may, however, be generally found, according to the form of the figure required, by the use of succeeding problems; and for the quadrilateral figure which occurs most frequently in practice, namely, the square, the following construction is more convenient than that used in the present problem.