All gowns are dresses, but not all dresses are gowns. In earlier English examples from the 16th century onward, the word “dress” referred to both men and women’s clothing ensembles. It might be modified by terms like evening, traveling, riding, and people can still see examples of this on invitations to certain events. An invitation might state that a function requires semi-formal dress or attire, for example. When an invitation states that the occasion is formal, women are expected to wear an evening gown. (source:wisegeek)
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