The nib of a pen can be said to be the most critical part of a pen. It varies from thin to thick. Generally, the most common NIB sizes are "B (broad thick), m (medium thick), FM (fine medium thin), f (fine fine) and ef (extra fine)", and from thick to thin is b > m > F > EF, Nib size each pen factory will also have some differences in specifications. Under the same size unit, the nib of Japanese pen will be thinner than that of European pen.

According to the usual writing habits, Westerners mainly use the M NIB when using pens, and choose the B NIB when signing. In Chinese writing, because the strokes are complex, you can generally use f or thinner EF nib.