Using elementary geometry and trigonometry, John Bryant and Chris Sangwin illustrate how to turn abstract mathematical models into physical tools that can solve real-life problems, from keeping a piston aligned in its cylinder to ensuring that automotive driveshafts rotate smoothly. They guide readers through paper-and-pencil reconstructions of mathematical problems and even provide instructions for how to build actual physical models. Combining problem-solving with stories from engineering history, How Round Is Your Circle? reveals some of the hidden complexities in everyday things.