If the line _AB_ is in a plane parallel to that of the picture, one end
of the line _AB_ must be at the same direct distance from the eye of the
observer as the other.
Therefore, in that case, _DT_ is equal to _D′T_.
Then the construction will be as in Fig. 7.; and the student will find
experimentally that _ab_ is now parallel to _AB_.[11]
[Illustration: Fig. 7.]
And that _ab_ is to _AB_ as _TS_ is to _TD_.
Therefore, to draw any line in a plane parallel to that of the picture,
we have only to fix the position of one of its extremities, _a_ or _b_,
and then to draw from _a_ or _b_ a line parallel to the given line,
bearing the proportion to it that _TS_ bears to _TD_.