During World War II, the "50 mission crush" cap was popular among aircrews of the United States Army Air Forces. Bomber and fighter aircrews had to wear headsets over their service cap during flight, so they would remove the stiffening wire from the cap. The headset would then crush the cap, which would eventually retain its crushed appearance. Since it took a good many missions to properly achieve the look, a so-called "50-mission crush" cap was considered a sign of a seasoned combat veteran. Current US Air Force regulations prohibit the wearing of such caps.