They've aged well since we introduced them back in 1910. Back then, we made them with rubber and a lace-up leather upper. We kept the classic look but swapped the rubber for an AeroForm shell made with polyurethane. This provides more flexibility and will keep feet warmer because the polyurethane won’t transfer cold to your feet like rubber can. The inside is smooth and seamless, adding to the overall comfortable fit and feel. AeroForm Technology, making a classic fit for the 21st century. Made of polyurethane, our AeroForm Shell wraps the foot for cushioning comfort and waterproof protection. It is lightweight and flexible and won't transfer cold to your feet.