See $QTDIR/mkspecs.

The list I get from the folder names in there seems to be incomplete. They do not include e.g. “win32” or “win64” without specifying a specific compiler.

However, I can still use

in a .pro file.

Where is that documented?

(Specifically, I am looking for a single common variable for “Windows” in general, whether it be 32 or 64 bit).

[EDIT: code formatting, please wrap in @-tags, Volker]

The platform variables resemble closely to the mkspec variants, so for example

• for a mac, it matches at lesat on unix, macx and one of (macx-g++, macx-g++40, macx-g++42)
• for windows it matches at least on win32 and one of (win32-g++ or win32-msvc20xx)

But there does not seem to be any “win” matching both win32 and win64, or is there?

Nope, there’s no “win” variable/scope. You can simulate this way:

This prints “win” too, without the first two lines it does not.

Okay, thanks!