Where are default fonts specified?

["A.R. (Tom) Peters" <tpeters@xs4all.nl>]

Does anyone know where in Debian default fonts for X are set?

After a recent apt-get upgrade apparently some config file as changed, to
the effect that an application that I use switched from a fixed width font
(courier 10) to a proportional font (Helvetica 12 apparently).

Setting the KDE fonts does not help: apparently that only applies to
window widgets etc., not to the content of the windows.

The program does not have a config file.  There is no reference for it in
my .Xdefaults .  In fact, I have no idea which X object settings it would
listen to: I tried entering plausible phrases into my .Xdefaults that
would set the font type for this application, but apparently it does not
use them.

Any hints will be appreciated.
--
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	Tom Peters