A tool for understanding a large circuit or system is to break it into its key functional blocks, in which the functionality of each block is represented by its transfer function. This is a type of approximation and abstraction in which other properties of the block such as its input and output impedances are not directly modeled. Rules are developed for manipulation and simplification of a complex block diagram to find its overall transfer function. Multiple examples are worked, including a state-variable filter that realizes a quadratic transfer function.