Designed by Campbell and Obstfeld to take frame and multifocal measurements, the City Rule is an optical ruler that enables the user to accurately measure the following: 
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Boxed Centre
Boxed Lens Size
Conventional Inset
Geometrical Centre of Segment
Geometrical Inset
Monocular Centration Distance (This is not a term or measurement defined in a British Standard)
Fitting Height
Segment Bottom Position
Segment Depth
Segment Diameter
Segment Height
Segment Size
Segment Top Position
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Supplied with instruction manual