Biangular coordinates

In mathematics, biangular coordinates are a coordinate system for the plane where ${\displaystyle C_{1}}$ and ${\displaystyle C_{2}}$ are two fixed points, and the position of a point P not on the line ${\displaystyle {\overline {C_{1}C_{2}}}}$ is determined by the angles ${\displaystyle \angle PC_{1}C_{2}}$ and ${\displaystyle \angle PC_{2}C_{1}.}$^[1]

The sine rule can be used to convert from biangular coordinates to two-center bipolar coordinates.