A very quick post to try to answer a query I had from a customer. Are the nibs on Jinhao fountain pens #5 or #6? Actually, the #5 or #6 refers to the diameter of the feed. The nib type (Fine, Medium, Gold, steel, titanium etc) that is fitted to the feed determines the width of the line produced by the pen, not the feed size. Even the material used in the nib affects line size.  So to summarise, #6 nibs are physically larger than #5 nibs but it does not necessarily follow that #6 nibs produce thicker writing than a #5.   

There is an excellent YouTube video that explains this much better than I can here: www.youtube.com/watch?v=0QaupR-DkDk