The module allows easy attachment of glassware with standard ground joint ST14. The specially shaped recess prevents the glassware from falling out.

Note on joint tolerances for ST14
Maximum outside diameter at the narrowest point of the cone joint: 17.2 mm (larger diameters are too wide for the holder and cannot be held)
Minimum outer diameter at the widest point of the cone joint: 18.0 mm (smaller diameters are too narrow for the holder and would fall out)