Mule shoes are shoes that are backless and almost always closed-toed. They can be any heel height (flat to high). The shoes are almost always worn by females. High heeled mule shoes were a popular indoor shoe in the 18th century. By the early twentieth century, the shoes were very often associated with prostitutes. However, in the early 1950s, Marilyn Monroe popularized the shoes and helped to break their substandard reputations. Similar shoes, called slides, were introduced in the early 1990s, except that slides are open-toed versions of mules.