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All sizing is listed in millimetres (mm) and follows the same convention.

The first number is the A measurement, or the widest width of the lens, regardless of the plastic or metal around it. It’s strictly the lens.

The second number is the smallest distance between lenses, or DBL. This is commonly referred to as the bridge.

The last number is the length of the temple itself. Extra measurements given are the B measurement, which measures the largest vertical depth of the frame and the ED, or effective diameter, which measures the widest straight line you can make on the frame. Oftentimes this is the same as the A measurement. Like the A measurement, both the B and ED are strictly measurements of the lens and not the plastic or metal surrounding the frame. 

For example,

54-18-135, B=38, ED=56

- This frame’s widest horizontal measurement is 54 mm.

- The bridge is 18 mm and the temple length is 135 mm.

- The deepest portion of the lens is 38 mm

- The widest straight line you can make on it is 56 mm