Let _abcd_, Fig. 22., be any square drawn in perspective. Draw the
diagonals _ad_ and _bc_, cutting each other in _C_. Then _C_ is the
center of the square. Through _C_, draw _ef_ to the vanishing-point of
_ab_, and _gh_ to the vanishing-point of _ac_, and these lines will
bisect the sides of the square, so that _ag_ is the perspective
representation of half the side _ab_; _ae_ is half _ac_; _ch_ is half
_cd_; and _bf_ is half _bd_.
[Illustration: Fig. 22.]