Cartridge-Converter: A cartridge-converter fountain pen can use either ink cartridges or an ink converter. The converter is a removable and refillable unit that replaces the cartridge. It allows users to draw ink from a bottle, providing more ink options than cartridges alone. Vacuum Filler: A vacuum filler fountain pen utilises a vacuum mechanism to draw ink into the pen's reservoir directly from an ink bottle. By compressing and releasing a mechanism on the pen, the vacuum is created, pulling ink into the pen. It offers a larger ink capacity than cartridge-converter pens and is relatively easy to refill. Piston Filler: A piston filler fountain pen employs a piston mechanism built into the pen's barrel. By twisting the end cap, the piston moves, creating a vacuum that draws ink into the pen from an ink bottle. This mechanism also provides a higher ink capacity compared to cartridge-converter pens and allows for easy refilling without the need for a converter or cartridges.