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Mule, a French word, is a style of shoe that is backless and often closed-toed. Mules can be any heel height - from flat to high. The style is predominantly (but not exclusively) worn by women.
The term derives from the Ancient Roman mulleus calceus a red or purple shoe worn by the three highest magistrates, although there is little indication of any structural resemblance.
High-heeled mules were a popular indoor shoe style of the 18th century, influenced by the patten, a backless overshoe of the 16th century. By the early twentieth century, mules were often associated with prostitutes.
In the early 1950s, Marilyn Monroe popularized the shoe and helped break its poor reputation.
Mules experienced some popularity in the 1950s and early 1960s, and were seen in the 1970s and early 1980s almost exclusively in the form of open-back Scandinavian clogs, but then re-emerged in the late 1990s, especially in its open-toed form (the "slide"), and began to dominate the shoe market for women.
For more information about mule shoes, please click on the following link: http://en.wikipedia.org/wiki/Mule_(shoe)