What's /d/ in threads URL? what about /t/?

'd' is for discussion/thread/topic whatever you want to call it
'u' is for user

As far as changing them, well nothing is impossible but why would you is a letter in a URL that important?

/d/ is tolarable somhow, but, Anyway, why not the tags instead of /d/?

Finally, /t/ is really awfull! Without /t/, we can have clean URLs for each subject to have something like stackexchange:

exmple.com/t/politics --> exmple.com/politics
exmple.com/t/economics --> exmple.com/economics
exmple.com/t/philosophy --> exmple.com/philosophy

Tags based URLs would break (or require redirecting) if the tags changed.