Cross-ratio
In geometry, the cross-ratio, also called the double ratio and anharmonic ratio, is a number associated with a list of four collinear points, particularly points on a projective line. Given four points A, B, C, D on a line, their cross ratio is defined as ( A , B ; C , D ) = A C ⋅ B D B C ⋅ A D {\displaystyle (A,B;C,D)={\frac {AC\cdot BD}{BC\cdot AD}}} where an orientation of the line determines the sign of each distance and the distance is measured as projected into Euclidean space.