The module allows easy attachment of glassware with standard ground joint ST29. The specially shaped recess prevents the glassware from falling out.

Note on joint tolerances for ST29:
Maximum outside diameter at the narrowest point of the cone joint: 31.4 mm (larger diameters are too wide for the holder and cannot be held)
Minimum outer diameter at the widest point of the cone joint: 32.8 mm (smaller diameters are too narrow for the holder and would fall out)