Research has shown that 1 in 30 passengers on a long-haul flight are at risk of Deep Vein Thrombosis (DVT). The use of Lifesystems graduated compression flight socks can improve circulation, relieving discomfort and helping to prevent circulatory problems such as Deep Vein Thrombosis (DVT). Your ankle has little muscle in it and requires more stimulus than your calf to speed up circulation. The knee length socks provide graduated compression to stimulate blood flow. size 19.5cm*42cm.