A parallel projection of a straight line is a straight line.
That a parallel projection of a parallelepiped is a parallelepiped.
A parallel projection of a pair of solids having a given ratio is a pair of solids having the same ratio.
A parallel projection of a curve, or of a surface of a given algebraical order, is a curve or a surface of the same order.
The results of parallel projection (see Projection, 17 and 18) may therefore now be used.