For me personally, I've often stated that my test for a pen and nib's fitness for purpose (as a 'general purpose' writer used in everyday applications) is whether I can write in traditional Chinese legibly within a 5mm grid, which is the most common spacing for line-grid and dot-grid journals and notepads on the market. But what does that actually mean, especially if I have to explain it to someone who is not familiar with the appearance and structure of the most common two or three thousand traditional Chinese characters?
I most commonly use the Chinese (as opposed to, say, Sanskrit) text of the Heart Sutra when I do writing samples, just as I typically use the text of the papal bull Inter Gravissimas for writing samples that use Latin-based characters. Let's look at the first character in the Heart Sutra: 觀.
觀 is a fairly commonly used character in Chinese outside of Buddhist sutras and texts; it appears in the Chinese words for objective and its opposite subjective, views, opinion, optimism, appearance, observe, sightseeing, Taoist shrine, and so on. Furthermore, the left half of that character is a component of quite a number of other common Chinese characters such as 歡, 灌 and 鑵.
In order for me to write 觀 (and any other Chinese character that contains the left half of it) legibly, I must be able to draw in sixteen parallel, (nearly) horizontal partitions inside the em-space, either deliberately putting ink in a partition or deliberately avoiding putting ink in one. A loose equivalent to that would be being able to draw within the height of the em-space eight distinct, parallel, horizontal lines that are cleanly separated. I also need to factor in enough spacing between two consecutive lines of writing to provide clear delineation, so let's make that nine horizontal lines (and eight 'white' lines of empty space separating them, preferably followed by a ninth as well).
Let's assume for now that the whitespace between two parallel lines of ink ought to be at least as wide as the lines themselves, in order to make their separation visually distinct. Therefore, each horizontal line of ink should nominally be no wider than one-seventeenth to one-eighteenth of the grid size being used. Using 5mm grid paper, that works out to approximately 0.28mm.
The line width requirement for parallel vertical lines inside the em-space is less stringent; seven to eight lines will generally suffice.
(It always makes me wonder why some people think architect nibs, which produce wider horizontal strokes than vertical strokes, are suited to Chinese and/or Japanese writing.)
Any pen – in combination with the specific ink and paper used, as well as limited by my own handwriting technique and hand-eye coordination – with which I cannot draw nine distinct parallel lines within the em-space is not fit for the purpose of 'everyday' writing for me. So that's the rule-of-thumb test I tend to use now, including when testing inks to see whether they're acceptable for use with certain pens I have.
Also keep in mind that 觀 is hardly one of the most complex or stroke-dense traditional Chinese characters. (It's not even as complex as one of the characters in my given name, which thankfully I don't have to write very often these days.) However, using a character such as 齉 at every turn as the standard for testing could be a little unreasonable; we don't talk or write about nasal blockage every day.
Interestingly, even though Japanese fountain pen manufacturers often use inscriptions of F, M, B, etc. on the nibs to denote their width grades, in Japanese the grades are actually denoted 細字 (small characters/ideograms/symbols/glyphs), 中字 (medium-sized characters), 太字 (large characters), etc. correspondingly. In other words, the user nominally selects a nib based on how large his/her handwriting is for a given purpose, and not how wide the nib tipping is or how wide a line it is apt to produce on the page.
I'd be happy to agree that, by most people's standards, writing Chinese hanzi or Japanese kanji in a 5mm grid, especially without whole lines of whitespace of that height in between lines of writing to aid legibility, is very small handwriting – 極細字 – which 'translates' to Extra Fine in terms of nib width grade (although I'm often able to do it with Japanese nibs that are classed as F nibs).
Edited by A Smug Dill, 17 February 2019 - 09:28.