The Gaming Den

It's been 17 years since I last used THAC0 and for the life of me can't recall how I ever understood it in the first place.

Could someone explain an easy method for calculating THAC0 in terms a non-computer-programmer with dyscalculia can understand?

I know I'm not the only one desperate for answers on this.

use a THAC0 table. I recently checked how it works to explain it to a friend, but already forgot everything except "table is useful" and "it's not complcated, just stupid"

To Hit Armour Class Zero. So if your THAC0 is nine, then for an opponent of AC 0 you need a 9+.

Then for every point your target's AC is above that, you subtract 1 (AC 1 is 8+, AC 2 is 7+) and for every point the AC is below that, you add 1 (AC -1 is 9+, AC -2 is 10+).

Alternatively, "Subtract the AC from your THAC0. That is the minimum number you need to roll."

Koumei wrote: Alternatively, "Subtract the AC from your THAC0. That is the minimum number you need to roll."

Thanks! I'll also delve into my old book and check the table.

I remember it caps at -10 AC so there was this 1-out-of-20 chance for a L1 Warrior to hit a difficult target, and for a level 20 Warrior it was.... 50%?

Without knowing the AC, it's the same to use as ...

Subtract the d20 roll from your THAC0. That is the best AC you hit.

eg: THAC0 20, you roll 17. 20 - 17 = 3. AC 3 is hit.

Your THAC0 is your TN, and the target's AC is added to your to-hit roll. Done.

JonSetanta wrote:I remember it caps at -10 AC

Not really.

-10 is the AC you get with the best armor (+5 full plate) and the best shield (+5 shield). Then you can subtract the Dex AC bonus, some item bonus like a ring of protection, etc, for a total AC far below -10.

so there was this 1-out-of-20 chance for a L1 Warrior to hit a difficult target, and for a level 20 Warrior it was.... 50%?

Except the warrior can have several bonus to hit: a Strength bonus, a magic weapon, etc (obviously, if the target has a +5 full plate and a +5 shield, his opponent should at least have a +5 weapon to be balanced).

GâtFromKI wrote:

JonSetanta wrote:I remember it caps at -10 AC

Not really.

-10 is the AC you get with the best armor (+5 full plate) and the best shield (+5 shield). Then you can subtract the Dex AC bonus, some item bonus like a ring of protection, etc, for a total AC far below -10.

We probably played it wrong, but we used the -10 as a hard cap. Once you got there, nothing pushed you past it. Dexterity, rings of protection, etc were all contingency stuff for when you didn't have your armor or shield for whatever reason.

violence in the media wrote:

GâtFromKI wrote:

JonSetanta wrote:I remember it caps at -10 AC

Not really.

-10 is the AC you get with the best armor (+5 full plate) and the best shield (+5 shield). Then you can subtract the Dex AC bonus, some item bonus like a ring of protection, etc, for a total AC far below -10.

We probably played it wrong, but we used the -10 as a hard cap. Once you got there, nothing pushed you past it. Dexterity, rings of protection, etc were all contingency stuff for when you didn't have your armor or shield for whatever reason.

There were definitely some monsters with better AC (one of the older dragons had -14ish, I think?), so a hard cap at -10 was definitely a houserule.

JonSetanta wrote: I remember it caps at -10 AC so there was this 1-out-of-20 chance for a L1 Warrior to hit a difficult target, and for a level 20 Warrior it was.... 50%?

A character with a THAC0 of 20 would need a 30 to hit AC -10. You'd only get a 1-in-20 in that case with 20s auto-hitting, not due to THAC0.

Emerald wrote:There were definitely some monsters with better AC (one of the older dragons had -14ish, I think?), so a hard cap at -10 was definitely a houserule.

Kind of. The actual rulebook said that AC was a scale from 10 to -10, full stop. That some elder dragons and shit had better than that was just an unexplained fuck you to the players.

angelfromanotherpin wrote:

Emerald wrote:There were definitely some monsters with better AC (one of the older dragons had -14ish, I think?), so a hard cap at -10 was definitely a houserule.

Kind of. The actual rulebook said that AC was a scale from 10 to -10, full stop. That some elder dragons and shit had better than that was just an unexplained fuck you to the players.

Maybe different printings of AD&D?

The -10 cap was for PCs. Monsters could go lower.

K wrote:The -10 cap was for PCs. Monsters could go lower.

Aha!

-10 only a PC cap in 2nd edition as well. PCs in 1st edition faced some wickedly complicated stacking rules on AC bonuses, hidden in the magic item descriptions mostly, but there was the odd one got down to -14 after Unearthed Arcana with full plate.

-8 best for 1e Wizards, easy to hit -10 in 2nd edition with the AC 0 bracers and robes.