Platinum ink cartridges are made specifically for use in Platinum and Nakaya brand fountain pens only.

Platinum ink cartridges are top quality. A stainless steel ball holds the ink from coming out until the cartridge is pushed into the back of the gripping section of your fountain pen. This serves a second purpose which helps meter the ink flow when the cartridge is opened and allows for priming when the pen is shaken.  

These cartridges fit all Platinum and Nakaya fountain pens. The range of colors is impressive and the ink quality superb.