Ball joints have the advantage that they have a certain freedom in lining up the connected glassware.
They are available in two different versions;

Grounded: Socket & ball are both grinded, these need vacuum grease.

With O-ring groove: Socket is flame polished & ball has a groove for an O-ring. O-rings can be made of NBR, FPM, Silicone or FEP (PTFE) coated silicone depending on what’s needed. No need for vacuum grease.

Coding of ball joints starts with KS (Kugelschliff) & Diameter of the ball / internal diameter of the glass tube so KS A/B.

Standard sizes are for instance 19/9, 19/12, 29/12, 29/15, 35/20 and 35/25.