Infiniti2000 said:
Really. More importantly, was that insult really called for? How did it help your post, Staffan?
It was more a matter of astonishment than insult, though if Ryan took it as an insult I wish to pre-emptively apologize for it. I just figured that someone who had written a book that's so acclaimed as Elements of Magic and been on ENWorld so long (it's not like this is the first time the topic has come up - we usually have a thread about it per month or so) would have known.
As for the other question posted in the thread: DR generally doesn't stack either, it overlaps (unless there's something that specifically says it stacks). If you have multiple types of DR, the proper way of applying it is to start with checking the one with the largest numeric value first, and then proceeding down the list until you find something that isn't penetrated.
Example: Let's say you have a titan (DR 15/lawful). This titan is also a 10th level barbarian (DR 2/-). He has a 10th level wizard buddy who has just cast
stoneskin on him (giving DR 10/adamantine).
He gets attacked by Adam, a 16th level monk, Betty, a 15th level monk, and Cyril, a 16th level fighter with a holy greatsword. Adam hits him, and does 18 points of damage (2d8+mods). First, we test DR 15/lawful - Adam has ki strike (lawful), so that's not a problem. We then test 10/adamantine - Adam has ki strike (adamantine) too, so again not a problem. Finally, we get to DR 2/-, which is a problem. The titan takes 16 points of damage.
Next, Betty takes a shot. She hits and does 12 points of damage (she only does 2d6+mods), and isn't as lucky as Adam. Again, we start with testing the 15/lawful - that's OK, on account of ki strike. Next we try the 10/adamantine - but Betty's hands aren't as poweful as adamantine yet, so she loses 10 points of damage from her attack, and only 2 points of damage goes through.
Last, Cyril gets an opportunity, and does 25 points of damage (2d6+mods+2d6 for holy). However, he runs into trouble even with the 15/lawful DR, so only 10 points get through.
As a side note, had Cyril wielded a flaming sword, that extra d6 would have been treated as separate damage which would not be subject to DR - but would be subject to energy resistance. Titans don't have that, but if he had been fighting a fiend of some sort that would probably have been absorbed.