All glasses have at least 3 important sizes on the lenses, bridge and temples. Some models have one size fits all, others have various sizes in order to meet the demands and face width of each client. See the following example:
Ex: Ray-Ban RX5184 2000 / 52-18-145

- The first number (52) represents the maximum width of the lens (mm);
- The second number (18) represents the length of the bridge (mm), ie the distance between the lens and which is situated above the nose;
- The third number (145) represents the total length of the temples (mm). Most commonly used in frames and not always in sunglasses.

Another situation is the case of single lens glasses (shades). Ex. RB3211 001/13 0126
The 0126 number is the width between the temples.