Certain serif fonts have distinctly different stroke widths for parts of certain letters (see WMVAU below) and there is a “correct” and an “incorrect” orientation. An example with Google Fonts Prata:
What is this called and why does the top one look “correct” and the bottom one “incorrect”?
It’s called “Stress” and it’s a remnant from when letters where hand-written by calligraphers and not yet printed by press.
The answer is really rather simple, and hails from the pre-printing
days when books where still written by hand. During these times a tool
called a calligraphy pen was used to produce hand-written volumes.
These pens had a flat tip that resulted in a trademark thick-thin
transition when the pen was angled a certain way.
After the printing press was invented
by Gutenberg, letters were no longer written by hand with calligraphy
pens. Instead, they were cast into metal representations of the
letters that with ink could be used to ‘press’ the letters onto paper
in an instant.
To give the ‘printed’ books the same authentic look as the
hand-written volumes, the metal cast ‘letters’ used by the printing
press were styled after the hand-written letters seen in volumes
written with calligraphy pens. Thus, the thick-thin transition of
stress is really just a throw-back to the days when books were written