Ground glass joints (GonG joints) are divided into four categories, those being, small, medium, conical and large. The large is also called scientific.

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.