A given pair of linear elements may have only one of the Angle, Parallel, or Perpendic constraints assigned to it. This is because Parallel and Perpendic are simply special cases of the Angle constraint. If you attempt to assign a Perpendic constraint to a pair of elements that is already assigned a Parallel constraint, the Perpendic constraint will replace it.

The action taken when you assign a Perpendic constraint to an element pair that is already assigned an Angle constraint depends on whether or not the Angle constraint is parameterized. If it is not, then the Perpendic constraint simply replaces it. If the Angle constraint is parameterized, however, the Perpendic constraint is not assigned. Instead, the value of the parameter is adjusted to 90° or 270° (effectively perpendicular). This is done so as not to invalidate any expressions that may reference the existing angle parameter.

Lines need not be in contact with one another to be constrained perpendicular. Following figure shows three different pairs of perpendicular lines.