Bispherical coordinates are a three-dimensional orthogonal coordinate system that results from rotating the two-dimensional bipolar coordinate system about the axis that connects the two foci. Thus, the two foci F 1 {\displaystyle F_{1}} and F 2 {\displaystyle F_{2}} in bipolar coordinates remain points (on the z {\displaystyle z} -axis, the axis of rotation) in the bispherical coordinate system.