Nib flexibility is a big topic and it’s likely that I will return to it again, but this is an outline of the subject.
The most rigid nibs are those that contain the most gold, quite simply. They’re made from thicker material, and include, for example, Conway Stewart’s Duro nibs, Mabie Todd’s Eternal and most Parker Duofold nibs. Those nibs were often housed in more prestigious pens, with a price to match. Though perfectly suitable for general writing, those rigid nibs were suitable for use with multi-part forms.
Next is the semi-flex. In my book, that’s a nib that expands by one nib-width with pressure, e.g. fine to medium or medium to broad. Semi-flexes are the commonest type of British post and pre-war nib. Most Conway Stewart nibs, I find, are semi-flex.
Flexible, to my mind, is a nib that expands two nib-widths with pressure – fine to broad or medium to double-broad. I most often find flexible nibs in Swans, Watermans and De La Rue Onotos.
The Super-Flex, then, expands more than two nib-widths. They’re not common, but in my experience they occur most frequently in Swans, Blackbirds and Watermans. My most flexible nib is a fine Whytwarth which will safely expand to five times its unflexed width.
Of course there’s more to flexibility than the width of the line that can be induced. There’s “return” – the alacrity with which the nib snaps back to its unflexed state when the pressure is eased. Without good return you can’t really achieve good calligraphic effects. Then there’s the amount of pressure that can be safely applied to induce flexing. Some of the larger Swans take quite a bit, others need a very delicate touch. It takes a lot of careful experimentation to explore the flex of your pen. Some excellent nibs can be easily cracked or sprung with too enthusiastic pressure – Waterman Juniors, Blackbirds and the thinner of the post-war Conway Stewarts fall into this category. Such damage can be repaired but as it’s expensive, it’s best avoided.
The ultimate, for many calligraphers, is the pen that is extremely flexible and takes little or no pressure to get there. Jocularly known as a “Wet Noodle”, such a pen can produce wonderful effects, if you have the ability to control it. I confess that I find it difficult. I use a fairly stiff full flex, slowly, for special calligraphic effects, and for daily use a semi-flex is enough. My current daily user is a Conway Stewart 85 that flexes from medium to broad with very little pressure, and it’s a real pleasure to use and imparts interest and character to my writing.