Glasses Bridge connects the left and right rim wires or connect directly to the lens
Some acetate bridge is placed directly on the nose.
Bridge width is the second number in the string (51-22-140) and is important in determining fit.
The bridge measurement is the distance between the lenses. If you have a narrow bridge or close set eyes you will want this number to be lower (15mm to 18mm) if you have a wider bridge or wide set eyes this number can be higher (20mm to 24mm).
Also consider the location of the bridge in relation to the browline of the frame. Frames with bridges that are even with the browline are longer if the bridge of your nose is higher on your face (above your pupils).
Frames with a lower bridge and a more arched browline are shorter if the bridge of your nose is lower on your face (approximately even with your pupils).