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Rubin's vase (sometimes known as the Rubin face or the figure–ground vase) is a famous set of ambiguous or bi-stable (i.e., reversing) two-dimensional forms developed around 1915 by the Danish psychologist Edgar Rubin.
The stereotypical example has a vase in the center, and a face matching its contour (since it is symmetrical, there is a matching face on the other side).