Angle ∠BAC might refer to any of four angles: the clockwise angle from B to C, the anticlockwise angle from B to C, the clockwise angle from C to B, or the anticlockwise angle from C to B, where the direction in which the angle is measured determines its sign (see Positive and negative angles). However, in many geometrical situations it is obvious from context that the positive angle less than or equal to 180 degrees is meant, and no ambiguity arises. Otherwise, a convention may be adopted so that ∠BAC always refers to the anticlockwise (positive) angle from B to
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