I’ve been using Emacs for over a decade and like all of us, I’m still learning things I didn’t know about it. Still, this really surprised me. Sure, maybe I don’t know about some obscure command or key binding but a modifier key?
At first I thought the person asking the question was simply misinterpreting something. Then I read the first couple of lines of the answer and thought, “Aha! It’s simply someone trying to convince us to use Alt instead of Meta.” But no. It turns out there really is an Alt modifier and it has nothing to do with the Alt key we all have on our keyboards and map to the Meta modifier1. It’s a completely different modifier that I, at least, had never heard of. It’s an artifact from the days when keyboards had both Alt and Meta keys. It’s seldom used these days but if you need it, the easiest way to specify it is with Ctrl+x @ a.
Perhaps I’m the last Emacs user in the inhabited universe to learn this but just in case I’m not, I’m sharing the information.
Footnotes:
Let this be a word of caution to those revisionists who are always fussing at us to be “modern” and call the Meta modifier the Alt modifier because the Meta modifier is most often mapped to Alt on today’s keyboards.