English: JoWo #6 ink feed in the foreground and its housing in the background. The finned structure at the rear half of the ink feed is intended for buffering fountain pen ink. Buffering is the capacity to catch and temporary hold an overflow of ink caused by other conditions than writing towards the nib. When a fountain pen nib receives such an overflow it will result in ink blobbing or dripping also know as burping.

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 5 mm or 6 mm in diameter respectively.