A device used in graphic arts photography and platemaking to determine the percentage reduction or enlargement an image requires, based on a ratio of the size of the original to the size it needs to fit. A proportional rule may, in one cofiguration, be two attached moveable disks, the size of the original on disk, the size of the reproduction on he other. When the original size is lined up with the reproduction size, a window in the middle of the scale indicates the percentage required to achieve the reproduction. Also known as a proportion scale.
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