Fountain pen nibs come in fine, medium and other sizes, but there’s no standard definition for those terms. A Japanese nib is usually a size finer than its European equivalent, for example, but there are exceptions all over the place. There are guides to a nib’s tipping size—the actual writing surface, measured in tenths of a millimetre—from Pen Chalet and Nibs.com, but they don’t necessarily tell the whole story. According to Pen Chalet, a TWSBI medium nib has the same tipping size as a Pilot medium, but my TWSBI Eco writes much thicker than my Pilot Metropolitan. The TWSBI nib might be wetter, and the ink might be too. And at the moment my Eco is loaded with a quick-drying ink that feathers a little on good paper. So it seems that there are other factors at play. I’ll figure them out as I go.