English: JoWo #6 ink feed inserted in its housing.
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.