A writing instrument having a thin and planar ink
cartridge adapts for placement and storage in narrow or flat spaces. The thin and planar ink
cartridge has a cavity-stabilizer inside the ink
cartridge to provide
mechanical stability of the cartridge and a stable volume of the cavity for ink storage within the cartridge. The writing instruments using thin and planar ink cartridges can be adapted for placement in wallets, checkbooks, notebooks, pockets, and in books and may function as bookmarks. The flat form of these writing instruments also provides a wide range of design possibilities to have variety of shapes resembling real or imagined objects and designs, and to
display advertising, reference, educational, and entertainment information.