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<blockquote data-quote="Libramarian" data-source="post: 6864307" data-attributes="member: 6688858">There is no arithmetic; just comparisons. If the die lands 11-20, the attacker compares it with their THAC0. If the die lands 1-10, the defender compares it with their AC. A negative THAC0 is extremely unlikely...maybe you can achieve that in like BGII Throne of Bhaal, but it's nothing to worry about in tabletop AD&amp;D play. I think you meant to ask about negative ACs? They're unfortunately pretty common in AD&amp;D at mid levels and up, which does reduce the utility of this system (it works great up to that point though). A negative AC would either apply as a negative modifier to the die roll, or a positive modifier to the attacker's THAC0.&nbsp;[MENTION=6790260]EzekielRaiden[/MENTION] here's an example:Consider THAC0 17 vs. AC 5. With the 2e PHB method you subtract the AC from THAC0, and that's the number you need to roll to hit.So 17 - 5 = 12 or better to hit. That's a 9/20 chance.With my method it's a hit if the d20 lands less than or equal to the AC, or greater than or equal to THAC0. So 1-5 or 17-20. 9 out of the 20 possible outcomes result in a hit, so the probability of a hit is the same.</blockquote>

[QUOTE="Libramarian, post: 6864307, member: 6688858"] There is no arithmetic; just comparisons. If the die lands 11-20, the attacker compares it with their THAC0. If the die lands 1-10, the defender compares it with their AC. A negative THAC0 is extremely unlikely...maybe you can achieve that in like BGII Throne of Bhaal, but it's nothing to worry about in tabletop AD&D play. I think you meant to ask about negative ACs? They're unfortunately pretty common in AD&D at mid levels and up, which does reduce the utility of this system (it works great up to that point though). A negative AC would either apply as a negative modifier to the die roll, or a positive modifier to the attacker's THAC0. [MENTION=6790260]EzekielRaiden[/MENTION] here's an example: Consider THAC0 17 vs. AC 5. With the 2e PHB method you subtract the AC from THAC0, and that's the number you need to roll to hit. So 17 - 5 = 12 or better to hit. That's a 9/20 chance. With my method it's a hit if the d20 lands less than or equal to the AC, or greater than or equal to THAC0. So 1-5 or 17-20. 9 out of the 20 possible outcomes result in a hit, so the probability of a hit is the same. [/QUOTE]

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