where a and b are the endpoints of the chord through p and q.
Isometries of the hyperbolic plane in the Cayley–Klein model are those projective
transformations that map the interior of C to itself. There are plenty of them; for example,
two triangles with equal corresponding sides are congruent, that is, they can be
mapped one to the other by a hyperbolic isometry.
Theorem 2 implies the following.
Corollary 4. The segments pq and rs on Figure 6, left, have equal length in the
Cayley–Klein model of the hyperbolic plane.