In the example shown below, the ratio is two. Specifically, the distance AC is one inch, the distance CB is half an inch, the distance AD is three inches, and the distance BD is 1.5 inches.
Harmonic division of a line segment is reciprocal; if points C and D divide the line segment AB harmonically, the points A and B also divide the line segment CD harmonically. In that case, the ratio is given by
which equals one-third in the example above. (Note that the two ratios are not equal!)
The points A, B, C, D divide the line harmonically as defined above precisely if the cross-ratio of the quadruple (A, B, C, D) is 1.
Harmonic division of a line segment is a special case of Apollonius' definition of the circle.