Why do mobile phones work when you're on a train? What happens when you pull a pop song apart into pure sine waves and play it back on a piano? And what did mathematicians have to do with the great pig stampede of 2012? The answer to each of these questions can be found in the triangle. Humans have been using triangles for thousands of years to build structures, measure the earth, make music, paint vanishing points, pot snooker balls and much, much more. But trigonometry is not a thing of the past - triangles underpin all of modern data technology. When someone Snapchats a photo, the light travels into the camera as electromagnetic sine waves, Fourier analysis compresses the image and then trigonometry is used to send the data to someone else's phone, when you listen to a track on Spotify, triangles remove the sounds which a human ear can't perceive and re-assemble the song so that it's small enough to stream. Triangles are the hidden pattern beneath the surface of the contemporary world. Join Matt Parker, stand-up comedian and author of the first ever maths book to be a number one bestseller, as he uncovers the secrets of trigonometry and shares extraordinary stories about the mathematicians, philosophers and engineers who dared to take triangles seriously.