Fine

What “Fine” means here

"F = Fine": about 0.4 to 0.5 mm, the everyday width and the most common nib in the case. He reads them honestly: "Marked 'F' for fine, we think it is a really generous fine. We would call it a medium" (SKU 6960). Watch for what he calls tweener pens, "a fine-medium or a medium-broad, that sort of thing" (search-by-nib-size).