[Illustration: Fig. 43.]
In general, inclined lines are most needed for gable roofs, in which,
when the conditions are properly stated, the vertical height of the
gable, _XY_, Fig. 43., is given, and the base line, _AC_, in position.
When these are given, draw _AC_; raise vertical _AD_; make _AD_ equal
to sight-magnitude of _XY_; complete the perspective-rectangle _ADBC_;
join _AB_ and _DC_ (as by dotted lines in figure); and through the
intersection of the dotted lines draw vertical _XY_, cutting _DB_ in
_Y_. Join _AY_, _CY_; and these lines are the sides of the gable. If
the length of the roof _AA′_ is also given, draw in perspective the
complete parallelopiped _A′D′BC_, and from _Y_ draw _YY′_ to the
vanishing-point of _AA′_, cutting _D′B′_ in _Y′_. Join _A′Y_, and you
have the slope of the farther side of the roof.
[Illustration: Fig. 44.]
The construction above the eye is as in Fig. 44.; the roof is reversed
in direction merely to familiarize the student with the different
aspects of its lines.