I'm surprised I haven't noticed this before. I was drawing up some letters and noticed letters like Z/N don't really follow the nib angle. So I got my PP out and wrote it just to be sure and sure enough the diagonal in /Z has little weight on the diagonal based on the nib angle and the stroke weight on /N has far more weight than lets say comparing to T's arm. It's as if the nib angle needs to change to accomodate that stroke weight. Pretty sure the change in nib angle is to compensate for weight but is this how roman letters were designed where nib angles changed between letters?
"Why do your Zs have thick diagonals?"
"They look better that way."
"Yup. Now start over."
I once wrote a pangram about someone having this exact realization:
When, as a type designer, one has more time to sit back and analyze what the whole effect is, at that point the idea emerges that the important part of Z should be a thick, not a thin stroke, despite the transgressive stress.
Goudy Old Style is an interesting face with regards to nib angle. Clever old Fred. Note that the majuscule and minuscule forms of Z and z are quite different. Mr. Goudy also made the horizontal strokes of E, F and especially T significantly thicker than those of the curved capitals. This was a hugely popular typeface for much of the 20th century, and the “funny” z and generally inconsistent stress were apparently not a problem!
(Image from Identifont.com)
Broad nib ductus has never implied a consistent angle, in Latin or any other script. A ductus is a pattern of movements to write a particular style of a script, often with a specific tool. The ductus of an individual style may centre on the movement of a broad tool at a more-or-less consistent angle, but still involve deliberate rotation for some features of some letters, or other manipulations such as tilting the nib (as in Arabic nastaliq) or actually outline drawing and filling some shapes (as in Arabic naskh).
From an earlier letter from Morison to Updike: