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Feeds are designed to feed ink from the inside of the pen to the nib. Feeds are usually specific to a type of nib. It must fit the underside of the nib and not leak. As an example, the feed for the Zebra Manga G nib is different from the feed for the Nikko Manga G nib. In some cases, a feed can fit several nibs. For example, the feed for the Nikko Manga G also fits the Tachikawa Manga G nib and the Principal nib. 

Feeds are the second most important part in a fountain pen. The nib is the most important part, but without a good feed, the nib won't work.