B joints are standard joint sizes in the UK and many other countries.
The full names of these joints are B29/32, B24/29, B19/26, and B14/23. The first number is the outer diameter (OD) in mm at the widest part of the ground glass cone, which is the same as the internal diameter (ID) in mm at the widest part of the ground glass socket (‘x’ in the diagram below). The second number is the length of the ground glass joint in mm (‘y’ below).
They have a standard taper (1:10 – for every 10 mm down the cone/socket, the diameter gets smaller by 1 mm).
The ‘B’ differentiates them from American ‘A’ joints, which have the same maximum diameter and taper but are longer – e.g. A29 is A29/42, and A24 is A24/40.
B joints are also sometimes referred to as ‘NS’ joints instead, so you may be familiar with that term – e.g. B29 (B29/32) is the same as NS 29/32, and B24 (B24/29) is the same as NS 24/29.