Fountain pen nibs come in different sizes. Some fountain pen companies have their nibs made by specialist nib makers like Jowo or Bock. These manufacturers make nibs in different sizes – nibs of the same size are interchangeable across manufacturers.

Some manufacturers have special lines e.g. Pelikan nibs made by Bock. They come attached with the feed and screw into the pen. Of course once could pull it out and push it into another pen but it will require some tuning to get it to write well.

Other manufacturers make their own nibs e.g. Parker, Waterman, Pilot, Sailor. Interchangeability of these nibs is possible but requires a bit of research to see which one will fit in a particular pen.

It is important to note that physically fitting a nib in the section does not necessarily mean it will work well. The feed needs to work with the nibs curvature (both radially and longitudinally). Some feeds (made of ebonite) can be set for different nibs using heat.

Here are some links that you might find interesting:

Bock Nibs
Jowo Nibs (on a commercial site)

– Salman