Angle-scanned interferometers are a class of ac interferometers in which the optical path difference is varied as a function of time by varying the incidence angle. The resulting interferogram has the properties of a hologram of a point source and may be used to determine the input energy wavelength and incidence angle. A new category of interferometer type is suggested, beam symmetry, which determines the output fringe pattern behavior as a function of input angle. An interference circle diagram is presented as an aid to understanding the characteristic response of symmetrical interferometers and angle-scanned interferometers. A number of angle-scanned interferometer examples are described in terms of the interference circle diagram.