A quick question about chemical formulae (Score 1)

The O9 would be toluene trinitrate (or actually 2-methyl-1,3,5-benzenetriol trinitrate),

Disclaimer: I am not now, nor am I ever likely to become, an organic chemist. So this is clearly going to be a dumb question. Okay, I'm fine with that, because now I have a chance to ask it: what on earth is that -1,3,5- for?

Big print version, please. And like the Cat says, "It's a magic door!" is an acceptable answer.