# Soddy circle

Inner and outer Soddy circles (red).

A Soddy circle is a solution to the three-circle form of the Apollonius
problem in which each of the given circles is tangent to (just touches) the other two.^{1} There are two Soddy
circles: the **outer Soddy circle**, which surrounds the three
given circles, and the **inner Soddy circle**, which is interior
to them. The inner Soddy circle is the solution to the four
coins problem.

## Reference

1. Soddy, F. "The Kiss Precise." *Nature*, 137: 1021, 1936.