English: Various fountain pen nibs.

Left to right: #5 Japanese flex nib, #5 extra fine Indian Serwex nib, #6 medium Chinese Jinhao nib, #6 fine OEM Iridium Point Germany nib of Asian origin, № 10/#6 Indian Ambitious semi-flex nib (the oblique cut nib slit is intentional for maximizing ink flow), № 10/#6 fine/medium chrome plated Indian Ambitious nib, № 10/#6 fine/medium gold plated Indian Ambitious nib, № 12/#6 fine/medium gold plated Indian Ambitious nib.
The nib bases and bodies of #5 nibs are curved for optimal fitting on 5 mm diameter feeds, #6 nibs for 6 mm diameter feeds and Indian № 10 and № 12 for 6.35 mm (¼ in) diameter feeds. The Indian № 10 and № 12 nibs often will fit on 6 mm diameter feeds. #6 and № 10 nibs are approximately 35 mm long, № 12 nibs are approximately 40 mm long.

Many nibs feature a breather hole. The breather hole has no actual function regarding controlling the ink or air flow. Its main function is to provide an endpoint to the nib slit and forestall inaccuracies during nib slit cutting. Adding distance between the breather hole and the nib tip adds elasticity or flexibility to the nib.