Thornton's Short Standard Fountain Pen Ink Cartridges, Green Ink, Pack of 36 Thorntons ink cartridges have a neutral PH, excellent coverage, smooth flow. The standard cartridges fit almost every cartridge/converter fountain pen on the market. The 12-pack features a plastic reclosable case that will help preserve the life of the unused cartridges. Each ink cartridge is 38.46mm long and 7.06mm in width.