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plumb line: a suspended thread with a weight at its end, indicating the vertical.
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plummet: a form of plumb line in which the 'line' and weight are a single rigid piece.
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polar dial: type of sundial, see article on the sundial.
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polyhedral dial: sundial with hour lines on various faces of a solid figure, see article on the sundial.
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prime vertical: celestial great circle passing through the east and west points and the zenith.
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