A duality is called a polarity if and for all points p and lines l of , respectively.
Important examples of polarities arise from bilinear forms. Given a three-dimensional vector space V over a skew field K, and a non-degenerate hermitean, sesquilinear form , the map defined by
defines a polarity of the projective plane over V.
An absolute point p of a polarity is a point p of with . Absolute lines are defined similarly.