Rotring nibs

I'm looking at a couple of Rotring 600s but the nib sizes have me
confused. What's the difference in the OB, B, OM? I figure the EF is
extra fine but the others I'm not sure of.

Thanks

O is for Oblique (cut at an angle). When you hold the pen to write, the
right side of the nib is a bit longer than the left side. B is for
Broad and M is for Medium. And you're right about the Extra Fine.

There's not usually an OF or OEF because on a narrow nib the slanted
cut doesn't make much difference.

EF - extra fine
F - fine
M - medium
OM - oblique medium
B - broad
OB - oblique broad


You also might see:
I - Italic (instead of a rounded tip, this pen writes with an edged
point for making thick and thin strokes, like calligraphy).

BB - double broad
BBB (or 3B) - triple broad
and so on.