Standard Taper Ground Glass Joints are interchangable.For either frosted, presicon-ground standard taper joints (same size),the inner ground surfaces (male joint) make a good seal with the outer(female joint)). The joint size is described by the form A/B. The first number A represents the outer diameter (OD) in millimeters (mm) at the widest point of the inner (male) joint and the second number B represents the length of the joint. For example, a 14/20 joint is 14 mm wide at the top (the widest part) and is 20 mm in longth. The joint has a standard taper 1:10, any two joints (male and female) with the same outside diameter (A)can fit into each other ,no matther how lenght the joints are. For example, a 24/40 (US standard size) joint on a condenser will fit into a 24/29 (European standard size)flask but the ground glass joint will protrude (40 - 29) 11mm into the flask.