All German ground glass joints are made of Schott Duran tubing by Herbert Arnold Robotics in Germany to extremely precise tolerances.

The female ground joint holds the male and is ground on the inside of the glass and is the outside piece.
The male ground joint is held by the female and is ground in the outside of the glass and is the inside piece.

ALL joints are ground with a consistent taper, meaning that as long as the first number matches, they will all fit one another. The first number is the width in millimetres of the top of the ground joint.

As an example, a 14/20 joint has a width of 14mm at the top and will fit any other 14/?? joints. The only difference is the length of the grind depending from which of the four categories they came from.

Periodically, some joints may be found to have small, black marks on the top of the joint. This is a product of the manufacturing process and the tools used to manufacture these joints. It should be noted that we won't be offering any exchanges, refunds, or returns because of these marks. By electing to purchase these joints, you're agreeing to these terms. Some glassblowers also experience a "wobbling" to the joints. Again, we're finding this issue in almost every brand on the market and for now, it's considered a "normal" manufacturing process defect.