The stoppers have an ergonomic grip that especially makes the opening and closing of volumetric flasks, mixing cylinders and reagent bottles easier and more secure. For optimal flask sealing the tapered bodies of the stoppers have moulded ridges. Coloured inserts in the different sizes of stoppers allow the easy identification of the corresponding ground (ns/TS) neck size. The colour identification system used for the inserts is identical to that used for the KECKTM interchangeable glassware clips.