This is an adapter for the Crayola Ultra-clean Washable Markers - Fine Line so they can be used in a Scribit. I used PolyMaker PolyFlex filament. The pen (10mm in diameter) fits snuggly into the adapter and the adapter is inserted directly into the Scribet's print head with no need for O-rings or tightening cap. They make preparing pens for the Scribet much simpler.