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Message: <blockquote data-quote="Ariosto" data-source="post: 4852806" data-attributes="member: 80487">The only additional "pain" I can see in THAC0 versus 3E-style AC is that it's simpler if someone knows both the attacker's THAC0 and the target's AC (which is the number added to the die roll).If you want to avoid arithmetic generally (and subtraction altogether) during play, here's a little trick:Write down a combatant's chance to hit base (unarmored) AC as number or less on d20 -- for instance, 11 for most (if not all) 1st-level characters without bonuses in any edition. Let's call that "attack factor" (AF). Note "defense factor" (DF) as the penalty to that; so, if base (descending) AC is 10, AC 4 gives DF 6. A bonus to hit adds to AF, while a penalty to hit adds to DF.Thus, modified AF 13 misses on a roll of 14+; a roll of 1-13 hits any lower DF. If AF exceeds 20, then you add the difference to the roll -- but short of that, you're not adding anything to rolls. That's simpler in play than 3E/4E, in which one must always add attack bonus to each roll (or else subtract it from AC) even if there's no other modifier.The drawback is that it involves some preparation if you're working with monsters written up with some other system in mind. In 3E/4E, though, the factors are pretty "naked". As an AC bonus is normally added to 10, just subtract 10 from AC; add attack bonus to 11."A stitch in time saves nine" -- adding just once rather than over and over.</blockquote>