a mirror which is placed on a pier, viz. a wall between two windows supporting an upper structure.
It is therefore generally of a long and tall shape to fit the space. It may be as a hanging mirror or as mirrored glass affixed flush to the pier, in which case it is sometimes of the same shape and design as the windows themselves. This was a common decorating feature in the reception rooms of classical 18th-century houses.
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the circle suited the Henrietta-Maria style of Celia’s head and neck, and she could see that it did, in the pier-glass opposite.
George Eliot. Middlemarch
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She [...] went up to the pier glass to see whether she really had done her hair.
Leo Tolstoy. Anna Karenina
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There remains a mirror, on the hall wall. If I turn my head so that the white wings framing my face direct my vision towards it, I can see it as I go down the stairs, round, convex, a pier glass, like the eye of a fish,
Margaret Atwood. The Handmaid's Tale (1986)
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In the mirror of the giltbordered pierglass the undecorated back of the dwarf tree regarded the upright back of the embalmed owl.