16. Directions may be referred either to the Horizontal or to the
Vertical. Referring them to the horizontal, we say of a certain
direction, that it is up-to-the-left, or up-to-the-right, or
down-to-the-left, or down-to-the-right, a certain number of degrees.
It may be thirty (30°), it may be forty-five (45°), it may be sixty
(60°),—any number of degrees up to ninety (90°), in which case we say
simply that the direction is up or down. Directions on the horizontal
may be described by the terms, to the right or to the left.
[Illustration: Fig. 5]
The method of describing and defining different directions from any
point, as a center, is clearly explained by this diagram.