Where was the term THACO first used?

[the_dunner]

I believe the abbreviation was first explicitly used as part of Battlesystem, the D&D miniatures war game which first came out in '85. I remember there were tons of Battlesystem harts for using THAC0, and that they made a big deal about explaining the term in that boxed set. Shortly after that came out, they started using it in Modules as well. I think that was because several of the following modules had portions that were intended to be played with Battlesystem.

 

[Lord Rasputin]

There was a discussion of THAC0 in the Dungeoneer's Survival Guide as being some new innovation. My guess is that some folks were including it in their products, some were not, and someone finally got around to discussing it in the DSG.

 

[T. Foster]

Sure, it's a shorthand reference to the DMG pp. 74-75 tables just like the original use of THAC0, only without the "repeated 20s" anomaly for 0-levels, low level magic-users and thieves, and monsters with less than 1-1 HD -- a 1st level cleric and a kobold technically both have a THAC0 of 20, but the cleric's THAC10 is 10 whereas the kobold's THAC10 is 11.

If you wanted to use THAC10 as a formula like THAC0, you'd subtract the target's AC from 10 and add that number to the attacker's THAC10 -- so a creature with THAC10 of 5 attempting to hit AC 4 needs a (10-4=6+5=)11; attempting to hit AC -3 he needs a (10+3=13+5=)18. Just remember when the result is over 20 that 21-25=20, 26=21, etc. So a creature with THAC10 8 attacking attempting to hit AC -6 would need a (10+6=16+8=24=)20.

 

[Melan]

This thread reminds me: am I the only one who thinks that THAC0 was a decent idea that didn’t live up to its potential because of a shoddily written formula? Let us consider the quotes supplied by Morrus and T. Foster:

The success of a character's or monster's attack is determined by rolling a 20-sided die. If the number obtained equals or exceeds the attacker's THAC0, the attacker has successfully hit a target with armor class 0. If the target has an armor class different from zero, the target's armor class is subtracted from the attacker's THAC0, and that number is what the attacker's roll must equal or exceed.

THAC0: This is an acronym for "To Hit Armor Class 0 (zero)." This gives the number that needs to be rolled on 1d20 for that being to hit Armor Class 0. To determine whether the attack hits another Armor Class, subtract that Armor Class from the THAC0 to see what number needs to be rolled for a hit. If a creature with a THAC0 of 15 attacks a PC with an Armor Class of 4, the creature needs to roll an 11 (15-4) to hit. if the PC is Armor Class -3, however, the creature needs to roll an 18 (15-[-3] = 15+3).

Both descriptions tell you what to roll to hit AC 0; to hit other numbers, you have to substract your opponent’s AC from the THAC0. This either means the DM will need to disclose confidential data, or handle the rolls himself.

In 90s Hungary, I had never seen the aforementioned methods used. Instead, it went something like this:

To figure out the success or failure of an attack, take the character’s THAC0 (figuring in to hit modifiers if applicable) and substract 1d20. The result is the Armour Class your attack hits. For example: a Fighter with 18 THAC0 rolls a 12 on the 1d20, hitting an AC of 6 (18-12=6). If the same fighter had rolled 18, he would have hit AC 0 (18-18=0), and if he had rolled 8, he would have hit AC 10 (18-8=10).

What we have here is essentially the 3e formula with reversed numbers and substraction. I am curious whether this method was commonly used in the USA - I always figured it was, but to my surprise, people are constantly quoting the other, less efficient formula... which, granted, was how the 2e PHB worded the rule.

 

[Plane Sailing]

We used THAC0 all the time in 1e, and from a very early point. It was very common in fan produced magazines in those days, and was trivially easy to use (I know that we used it prior to charts which had six 20s on them, whenever those were introduced. Does that mean we were using it with our OD&D+greyhawk+blackmoor+eldritchwizardry games?).

If you had THAC0 of 15 and you rolled a 12 you just said "I've hit AC3" and the DM told you whether you had hit or not. If you rolled a 20 you said "I've hit AC-5".

Like I say, it was trivially easy to use in play. Maybe it was just difficult to express the formula in words when TSR tried to write it down

Cheers

 

[Melan]

Plane Sailing: coincidentally, you just answered my query. Thanks!

 

[Bagpuss]

We used Melan's method, but then I'm from the UK.

 

[diaglo]

for our campaign there was no AC 0 for PCs prior to UA. we used weapon vs armor type.

monsters have AC 0 or less. but plate and shield was the best you could get at AC 2 for a PC. magic makes it harder to hit... but it doesn't change your armor type. dex bonus helps but again doesn't change armor type.

it wasn't until they introduced field plate and full plate that AC dropped below 2.

 

[Henry]

It's a testament to the lax attitude towards the AD&D rules at TSR in those days that the "repeated 20s" paradox isn't mentioned and, for instance, the kobolds on p. 43 of that book are listed with THAC0 20 with no asterisk or footnote indicating that this is actually the 2nd, rather than the 1st, 20 on the table (and thus calculations To Hit other ACs should be based on 21 rather than 20)...

I don't know if I'd call it so much "lax attitude" as that the use of said rules in-house had actually changed by that point; seven years of water had gone under the bridge since the release of the DMG, and as Plane Sailing said, fanzines, as well as casual gamers, had been using the term since then. I know the guys I gamed with in the early 90's were using it before 2nd edition had even come out (in the mid-to-late 80's). 2E was probably being tossed around in early stages in 1986 anyway, so the idea that something originating in fanzines that was easier to use in play was plausible. (For me personally, it was always easier to use a formula than a chart.)

 

[Piratecat]

The THAC0 has been made in mexico since pre-colombian times...

So true. In fact, my uncle's best friend was attacked by a chupathac0 while visiting Mexico in the early '50's. The press hushed it up, though.