A converter "converts" a cartridge pen into a piston filled pen. These are for use in all Franklin-Christoph and Piper fountain pens that are labeled as "cartridge/converter" filled. This is the standard international standard size, for use in pens that handle the small or large international cartridges. Even though this is the most standard / commonly used size converter, check with particular brands to be sure this is compatible. Instructions: place converter into section just as you would a cartridge. Twist piston all the way down. Submerge nib into ink bottle and slowly twist piston up to pull ink into the chamber. You may need to push ink out and repeat to get a full load of ink. Be sure to clean nib and section thoroughly before inserting back into the cap. An alternative method is to submerge the end of the converter into the ink, fill, and then clean the end and plug into the FP section. These converters will hold approx .7ml of ink, not counting what goes in the feed/nib if filled that way.