THAC0

"rhymes with Blacow"
now I just need to know how to pronounce Blacow

Sacrosanct said:

Officially AD&D 1e was like @Mannahnin said, however our group, and every group I saw, ignored that and pretty much used the THAC0 method from as early as I can remember (I started in 1981). Just like we ignored a bunch of other official rules like weapon vs. armor. I think it really is divided among two groups back then:

1. wargamers who moved on to D&D, which seemed to use a lot more of those tables and rules because they were more familiar with them
2. gamers like me who started after D&D came out, and saw all those tables as impediments to the speed of game play and fun, so we only used the rules we wanted and streamlined everything else that we could.

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Yea, but thac0 as used didn’t include the repeated 20s. How could it?

darjr said:

Yea, but thac0 as used didn’t include the repeated 20s. How could it?

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It doesn't. That's the part we ignored. The repeated 20s.

AlexofBarbaria said:

"rhymes with Blacow"
now I just need to know how to pronounce Blacow

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That seems like a really random name to use, but it refers to Glenn Blacow, a contributor to Alarums & Excursions who also had his own APA, The Wild Hunt. He wrote an extremely influential article about the four main approaches to RPGs (roleplaying, power-gaming, wargaming, and storytelling). I'm pretty sure he pronounced his name BLACK-oh, but I'm not absolutely certain.

THAC0 itself functions the same in 1E and 2E. Only the to-hit tables underlying it might be different in that they featured no repeat 20's in 2E. For 1E only some of the underlying tables could result in a THAC0 of 20 which might not be the first of the repeating 20's, meaning that a player still needed to understand how those repeating 20's worked (total of die roll and bonuses amounting to at least 20, or needing a natural 20 on the die regardless of bonuses). But THAC0 itself was determined the same way - by looking at the actual TABLE - whatever that table was. And the 1E DMG, which was the first appearance of THAC0, nonetheless LISTED those 20's without notation as to whether they were the first in the sequence or not. The fact that 2E didn't include the repeats on the actual tables doesn't change how THAC0 worked.

Nikosandros said:

Nice, I didn't know that. Personally, when I re-started playing AD&D (about 20 years ago, now), I "smoothed out" the attack and saving throw tables and, later, I did the same for OSE.

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I think that was pretty common, probably in part because Gygax specifically provided it as an option right under the Fighters' attack matrix on page 74 of the 1E DMG.

"Special Note Regarding Fighters' Progression: This table is designed to allow fighters to advance by 5% per level of experience attained, rather than 10% every 2 levels, if you believe that such will be helpful in your particular campaign. If you opt for a per level advancement in combat ability, simply use the table but give a +1 "to hit" bonus to fighters who attain the second level of experience shown in each group of 2 levels, i.e.1-2, 3-4, etc. You may, of course, elect not to allow per level combat advancement."

darjr said:

In the case for the A&E article I don’t think 1. is correct, it looks like they did away with the table and used thac0 just like later folks do.

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The A&E article is from 1978, so it pre-dates the 1E attack tables. It's a simplification of the OD&D attack tables, which didn't have the repeating 20s.

darjr said:

In fact I’d argue that the very calculation of a thac0 doesn’t include the repeated 20s at all.

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Right. I agree. THAC0, by it's nature, includes no repeating 20s. Which is how it differs from the 1E attack tables.

Man in the Funny Hat said:

THAC0 itself functions the same in 1E and 2E. Only the to-hit tables underlying it might be different in that they featured no repeat 20's in 2E. For 1E only some of the underlying tables could result in a THAC0 of 20 which might not be the first of the repeating 20's, meaning that a player still needed to understand how those repeating 20's worked (total of die roll and bonuses amounting to at least 20, or needing a natural 20 on the die regardless of bonuses). But THAC0 itself was determined the same way - by looking at the actual TABLE - whatever that table was. And the 1E DMG, which was the first appearance of THAC0, nonetheless LISTED those 20's without notation as to whether they were the first in the sequence or not. The fact that 2E didn't include the repeats on the actual tables doesn't change how THAC0 worked.

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I suppose that comes down to what you mean by THAC0. Are we just talking about the value, or about the whole algorithm of how you determine your number needed to hit? ie: THAC0 as a system, rather than just a value. Because when we're talking about how THAC0 is used in 2E, and by folks using it as a simplification of 1E, it's usually both.

While yes, you look up your number needed To Hit Armor Class 0 on a table either way, the attack algorithm in 2E is functionally that you subtract the AC from your THAC0 to find out what number you need to hit. And if that number is over 20, you just revert to the auto-hit on a 20 rule.

If I make a 1st level Fighter in 2E I look up the Calculated THAC0s chart on page 91 of the PH and I see that my THAC0 is 20. So when I run into an AC5 enemy I know I need a 15 or better to hit. But if by terrible mischance I run into an enemy with AC -5 that would be a 25, except for the auto-hit on a 20 rule.

If I make a 1st level Fighter in 1E I can look up the Attack Matrix for Fighters (et al.) on page 74 of the DMG and see that same 20 to hit AC0, but if I run into that AC -5 enemy my target number is instead a 21, which I can't hit at all unless I have a modifier to apply.