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