User blog comment:Nayuta Ito/Guessing the definition of PGN from Psi's Current Value Of PEGG/@comment-30754445-20170915111150/@comment-30754445-20171126004205

I've used PGN in exactly one other place before: On the "My Number is Bigger (everyman needed)" game we've held on xkcd. You can actually see the notation evolving as the game progressed.

The base 9 equivalent is unique to my PEGG blog, though (there's a hint of the idea near the end of that xkcd thread and I've even used the words "base 9", but the notation itself never appears).

BTW that game is a wonderful treasure trove of googology for beginners... at least up to ωω. Unlike all other "my number is bigger" games, that one actually requires the players to explain in depth every single thing they're doing (other games had a similar "your number must be understandable" clause, but it was never enforced). It also proceeded much more slowly, so people can look around and enjoy the view (it took over 1500 posts to get to ωω).