If you’re an ink cartridge user, here’s a neat trick for carrying around a spare one in your pen so you can easily replace it on the go. Basically, just take a small standard international cartridge (often referred to just as a standard international cartridge), flip it around, put it in the body of the pen, and then install a cartridge just like normal in the grip of the pen and assemble it. You then have a spare to carry around in your pen for when you cartridge runs out and you’re in the middle of a meeting or traveling somewhere that you don’t have ink handy. This trick works for all pens that accept a standard international cartridge/converter, as two short standard international cartridges back to back is almost exactly the length of a converter.
I doubt this will work for brands with proprietary cartridges, most of them tend to be longer and I can’t see them fitting doubled-up in a pen (think Lamy, Pilot, Platinum). But pens like Monteverde, Jinhao, Edison, J. Herbin, Delta, Pelikan, Stipula, Omas, Mont-Blanc and others could all benefit from this little trick. Just make sure you’re keeping a spare of the same ink in both cartridges, otherwise you’ll get an interesting blend!
Be sure to check out the Goulet cartridge/converter guide, it’s not exhaustive but it shows a number of popular brands and what type of cartridges they take. Popular ink brands that make standard international cartridges include Diamine, Private Reserve, Monteverde, Pelikan, Mont-Blanc, Kaweco, Montegrappa, Omas, Delta, and many others.