English: JoWo #6 nib unit assembled. The tip of the nib is made out of an alloy containing Osmium, Ruthenium and Tungsten. JoWo nib units are configured for (disposable) standard international ink cartridges, meaning they have a 2.5 mm diameter ink nipple at the end for attaching such cartridges. Alternatively a standard international sized ink converter can be attached that can be filled and refilled with bottled fountain pen ink.

The business end of a fountain pen nib unit consists of three parts - the nib housing, the ink feed and the nib itself. It is usual that the whole assembly is mounted inside a casing called the nib or grip section, via a corresponding thread on the housing.

Nibs come in a variety of sizes, the most popular ones being size 5 (or type 5, or #5) and size 6 (or type 6, or #6). They are so called because they are designed fit around an ink feed which is 5mm or 6mm in diameter respectively.