UGG Classic Cardy Ugg boots (sometimes called uggs) are a style of unisex sheepskin boots lined with wool on the inside and with a tanned outer surface. Ugg boots often have a synthetic sole, although this is not universal. Heights range from around the ankle to above the knee, like the UGG Bailey Button, and they are available in a range of different colors as both slip-on and lace-up varieties of UGG Classic Tall. They are produced by a number of manufacturers. The natural insulative properties of sheepskin give thermostatic properties to the boots: the thick fleecy fibers on the inner part of the boots allow air to circulate and keep the feet at body temperature. This means that ugg boots can be worn without socks even in relatively cold weather. The ugg boot style is believed to have originated in Australia or New Zealand, although the exact date is uncertain — they have been, at different times, identified with pilots in World War I who are said to have worn them for warmth in unpressurized planes, shearers in rural Australia during the 1920s, like the UGG Classic Cardy, and surfers and competitive swimmers who wore them in the 1960s and 1970s for keeping warm while out of the water. In the early 2000s, they became a fashion item in the United States, leading to increased global sales.