Well, as I said Jester, I was looking at it from a book perspective.The position of WotC was that it was Core and non-optional. And even things like the campaign settings were assumptions.
But, more tellingly, how many optional rules can you name? How many variant rules? How easy is it to modify the rules for other setting or non-standard play styles?
For example, in 4E, you can play the game just as well with or without Martial Power 2. Or Arcane Power. Or the PHB3.
It's true (as I said) I wasn't considering individual rules within those books. And yes, you are correct, if you look at it that way, there isn't much in the way of variant rules and options.
But that's not how I was considering it when I said D&D has always been modular.
And frankly, I don't care what WotC considers modular or not; to me, if you can play without it or with it, without completely changing your game, then to me, it's modular.




