Inscribed angle its intersection points equals half of the central angle subtended by the chord. See also Tangent lines to circles. The inscribed angle theorem is used in many proofs of elementary Euclidean geometry of the plane. A special case of the theorem is Thales' theorem, which states that the angle subtended by a diameter is always 90°, i.e., a right angle. As a consequence of the theorem, opposite angles of cyclic quadrilaterals sum to 180°; conversely, any quadrilateral for which this is true can be inscribed in a circle. As another example, the inscribed angle theorem is the basis for several
Perceived visual angle Ono & Mapp (2006) noted, it unfortunately has been ignored in many current theories of size perception, and size illusions. The object's other perceived endpoint, A′, has a perceived direction "d′";, about which the observer might say "it appears toward a higher elevation than point B′." The difference between the two perceived directions () is the perceived visual angle "θ′", also called the perceived angular size or apparent angular size. It is not easy to quantify "θ′". For instance, a well-trained observer might say that point A′ "looks about 25 degrees higher" than B′, but most cannot reliably say how