TWSBI ECO Transparent Blue FOUNTAIN PEN Pen cap is able to post onto the back of the pen Piston Filler Fountain pen. Simply unscrew the end cap, dip into ink, screw back the end cap, and your done! Ready to write!! Pen cap has an inner cap to insure a perfect seal when stored away. Made for the beginner who just wants to try out fountain pens, all the way to the experts. A good everyday worker.