Hi Ansley and Karl,
Thank you for the explanation about "default", "serif", and "sans".
I've found that the "default" font is "LucidaGrande" on Qt (command-line plotting) and "Helvetica" on pango ( "-nodisplay" plotting). This is with PyFerret v7.6 on macOS 10.15.5 . According to what I found on the Internet, LucidaGrande has been the default font for recent versions of macOS.
You can find out what fonts are used on Acrobat Reader with "File" > "Properties" > "Fonts".
Off-topic: Here are some thoughts on the system fonts I've tried so far:
Arial and Helvetica: Have nice clean lines and are very suitable for scientific plots, except that their "hyphen-minus" symbols are too stubby to be used as a minus sign.
Courier: Is clean and suitable for scientific plots. Its "hyphen-minus" symbol is very long and suitable for the minus sign. Except that the result looks as if it came from the 1960s, when a contraption called "typewriter" dominated.
LucidaGrande: Is simple and clean, perhaps a bit childishly simple. Its "hyphen-minus" symbol is thin and long-ish, suitable for the minus sign.
Ryo