Since _ABCD_, Fig. 20., is a square, _BAC_ is a right angle; and as
_TV_ is parallel to _AB_, and _TV′_ to _AC_, _V′TV_ must be a right
angle also.
As the ground plan of most buildings is rectangular, it constantly
happens in practice that their angles (as the corners of ordinary
houses) throw the lines to the vanishing-points thus at right angles;
and so that this law is observed, and _VTV′_ is kept a right angle, it
does not matter in general practice whether the vanishing-points are
thrown a little more or a little less to the right or left of _S_: but
it matters much that the relation of the vanishing-points should be
accurate. Their position with respect to _S_ merely causes the
spectator to see a little more or less on one side or other of the
house, which may be a matter of chance or choice; but their
rectangular relation determines the rectangular shape of the building,
which is an essential point.