I always wondered, is there an actual POSIX documentation or spec out there? The last time I researched it, it seemed that POSIX is a number of documents behind

I always wondered, is there an actual POSIX documentation or spec out there? The last time I researched it, it seemed that POSIX is a number of documents behind a massive paywall. Yet, so many people seem to know what is and isn't POSIX compliant, that it just seems unlikely that POSIX is locked behind a paywall.