Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Inversive distance (usually denoted as ) is a way of measuring the "distance" between two non-intersecting circles and . If and are inverted with respect to a circle centered at one of the limiting points of the pencil of and , then and will invert into concentric circles. Apollonian circles are two families of circles such that every circle in the first family intersects every circle in the second family orthogonally, and vice versa. These circles form the basis for bipolar coordinates. They were discovered by Apollonius of Perga, a renowned Greek geometer.