Pens that are refilled with cartridges with ink usually use cartridges in the international standard format. However, there are exceptions and are not a matter of course, so you should check which cartridge your particular pen uses. This is the small format and in the vast majority of pens you can insert two into the pen body. To always have one in reserve. The advantage of small ones is that less is wasted if the pen is left lying around for a period. Mr Retro Pop are very nice pens from Pilot that are in our range and that these cartridges fit into.