If the line _AB_ is in a horizontal plane, the vertical distance of one
of its extremities must be the same as that of the other.
Therefore, in that case, _AC_ equals _BC′_ (Fig. 6.).
And the construction is as in Fig. 8.
[Illustration: Fig. 8.]
In Fig. 8. produce _ab_ to the sight-line, cutting the sight-line in
_V_; the point _V_, thus determined, is called the VANISHING-POINT of
the line _AB_.
Join _TV_. Then the student will find experimentally that _TV_ is
parallel to _AB_.[12]