This joint has an o-ring groove to accept the listed o-ring size and provides a grease-free vacuum seal when used with CG-150 clamps. Each joint is supplied with a Viton o-ring of the listed size. Clamps and additional o-rings are sold separately.
The interior diameter of the tubing matches the size - for example, a #15 O-ring joint has a 15 mm diameter. For a discussion of o-ring joints, see the Joints entry in the Glassware Gallery.