Now that summer is coming to an end and fall is just around the corner, now is a good time to swap out those colorful water-ready timepieces for something more restrained. Step into the world of ultra-thin watches, where mastery is measured in how few millimeters a case height reaches. From a simpler time and date version to a complex tourbillon and a few surprises in between, here are four incredible ultra-thin watches to show us how it’s done.