Kaweco standard converter fits all Kaweco long fountain pens. Instead of two ink cartridges, the converter is inserted into the barrel of the fountain pen.
The standard twist converter mechanism makes it easy to fill up ink from an ink bottle. 
Kaweco's standard converters are available in a whopping 10 colors, matching the colors of Kaweco's inks. These colored converters look great inside the transparent Kaweco Student and Kaweco Perkeo fountain pens!
A major advantage of converters is above all the sustainability aspect: Unlike conventional ink cartridges, which end up in the trash after the ink has been used, the converter can be permanently reused and refilled again and again.