Low Clamp 31.8

The front derailleur mounts to the frames seat-tube via a band clamp that is located below the main pivot points of the front derailleur hence the name "low" clamp. Whereas a high clamp positions the band clamp above the main pivot points of the front derailleur.
In this case the inside diameter of the front derailleurs band clamp measures 31.8mm in diameter so the outside diameter of the frames seat-tube should also measure 31.8mm to ensure a snug fit. A securing bolt is used to connect the band clamp together and secure the front derailleur to the frame.
A 31.8mm band clamp is only capable of fitting onto a frames seat-tube which is 31.8mm or smaller. If smaller then an adapter/shim may be used to fill the excess space.