Under idealizations about the nature of the path taken by sounds to each of a listener's two ears, and the shape of the listener's head, it is shown that both the azimuth and the range of a sound source can be expressed in terms of the rate at which certain sound stimulus parameters change when sound-source azimuth varies. It is further shown that these rates of change can be approximated by a listener if he rotates his head on its vertical axis in ways, and to extents, that are appropriate. The conclusion, then, is that a listener can calculate the azimuth and range of a sound source wholly on the basis of his interaction with the sound stimulus, without any prior knowledge of the sound source. The problem of localization error is discussed, particularly in relation to the extent to which our idealizations depart from reality, and our results are briefly related to those of other researchers in the field of sound-source localization.