My pen takes International size cartridges. What are these?

"International" size, "International Standard" or just "Standard" size cartridges are generally used in pens that do not take proprietary cartridges (ie not Lamy, Parker, Aurora, Sheaffer, Pilot/Namiki, Sailor or Cross). Standard size cartridges are 38mm (1.5in) long and taper towards the part that goes into the nib unit (grip section). Capacity of standard-size cartridges is approximately 0.8ml. They are made by, for example, Diamine, Herbin, Kaweco, Monteverde, Private Reserve, rotring, Visconti, Waterman and Yard-O-Led.
See also our Cartridge and Converter Compatiblity page.

The picture below shows Diamine standard size cartridges.