It does this through the use of a series of narrow channels or "fissures" that run down its lower edge. As ink flows down these fissures, air is simultaneously allowed to flow upwards into the reservoir in an even exchange of volumes. The feed allows ink to flow when the pen is being put to paper but ensures ink does not flow when the pen is not in use. The feed makes use of capillary action; this is noticeable when a pen is refilled with a brightly coloured ink. The ink is taken up and into the feed by way of capillary action (and is often visible in clear demonstrator pens), but is not dispensed onto the paper until the nib makes contact.

How the feed is shaped may determine the wetness and flow of a particular pen. For this reason, feed material alone and its surface roughness may have a significant effect on the way two pens of the same nib size write.

Pen feeds are crucial to preventing ink from dripping or leaking. Feeds often feature finned structures intended for buffering fountain pen ink. Buffering is the capacity to catch and temporarily hold an overflow of ink, caused by conditions other than writing. When a fountain pen nib receives such an overflow it will result in ink blobbing or dripping also known as burping. A pen with a misconfigured feed might fail to deposit any ink whatsoever.

Fiber feeds

Some fountains pens use a fiber wick underneath the nib. They often have a plastic part that looks like a feed that is only used to hold the fiber wick in place and does not assist with ink flow. The mechanism of action is like a felt pen, just with a fountain pen nib on top of it. The fiber feeds offer plenty of ink flow and can stay wet for extended periods. Cleaning fiber feed pens can require longer soaking in water.