The heart of the chapter: how to differentiate a composition of functions, such as sin(x2)\sin(x^2), exe^{\sqrt{x}}, arctan(1/x)\arctan(1/x). The circle diagram method represents the composition as a chain of links, calculates the derivative of each link with geometric symbols as placeholders, and then multiplies and substitutes. It is the visual version of Leibniz’s chain rule.