Mules are high-heeled shoes with open toes and no heel-strap. They're often made with nothing but a single strap across the toes used to hold the foot onto the sole.
Similar shoes include clogs and pantolettes, whose soles are both made of wood; sabots, which have a closed toe box; and slides, which have low heels.
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While they were first popularized in the eighteenth century where they generally was decorated with embroidery or buckles, the mules most vivid in modern imagination are graced with marabou.
concepts, but not verbiage, from the alt.sex.fetish.fashion faq and from costumes.org.