Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Elliptic cylindrical coordinates are a three-dimensional orthogonal coordinate system that results from projecting the two-dimensional elliptic coordinate system in the perpendicular z-direction. Hence, the coordinate surfaces are prisms of confocal ellipses and hyperbolae. The two foci F1 and F2 are generally taken to be fixed at a and + a, respectively, on the x-axis of the Cartesian coordinate system.