The quantity p s - r q obtained by letting x | = | p X + q Y y | = | r X + s Y in a x^2 + 2b x y + c y^2 so that A | = | a p^2 + 2b p r + c r^2 B | = | a p q + b(p s + q r) + c r s C | = | a q^2 + 2b q s + c s^2 and

quadratic | quadratic invariant