I understand that there are differences between the way Armor Class works in AD&D (first edition) and Basic D&D (the boxed sets, Rules Cyclopedia, etc.), but I don't have the books to check it out. My understanding is that one starts at AC 9 and the other starts at AC 10, but I don't know which is which. I'm going to be running a campaign using Labyrinth Lord and the Advanced Edition Companion with old modules from both versions of D&D. Will I need to adjust AC? Are there any other armor differences I should know about?
[RPG] AC differences between AD&D 1E and Basic D&D
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In general in play they were ignored or just treated as an abstract language with no further comment.
As to where they came from, here's an answer from Gary Gygax on Dragonsfoot!
As D&D was being quantified and qualified by the publication of the supplemental rules booklets. I decided that Thieves' cant should not be the only secret language. Thus alignment languages come into play, the rational [sic] being they were akin to Hebrew for Jewish and Latin for Roman Catholic persons.
I have since regretted the addition, as the non-cleric user would have only a limited vocabulary, and little cound [sic] be conveyed or understoon [sic] by the use of an alignment language between non-clerical users.
If the DMs would have restricted the use of alignment languages--done mainly because I insisted on that as I should have--then the concept is vaible [sic]. In my view the secret societies of alignment would be pantheonic, known to the clerics of that belief system and special orders of laity only. The ordinary faithful would know only a few words, more or less for recognition.
In other words, it was supposed to be more like religious languages, but wasn't really well thought through. It disappeared in Second Edition and was not missed.
Moldvay is great... in part because it's short. Tom and I spoke at some length about the 'tack' he would take. I later used a lot of ideas that he omitted because he just didn't have room. The following will address the BECMI treatment, being the most detailed expansion of Moldvay's data. At this distance (almost 30 years), most players consider the two nearly equivalent. (With apologizes to the ardent and more discriminating fans of either edition...)
Special Abilities are defined in BECMI, and summarized in the Rules Compendium (RC). The majority are Special Attacks, which include:
Acid, Blindness, Charge, Charm, Continuous damage, Disease, Energy drain, Paralysis, Petrification, Poison, Spell ability (1 per 2 spell levels), Swallow, Swoop, Trample
Some Special Defenses (Immunity to normal weapons, Spell immunity) are also Special Abilities.
Although the carrion crawler merits only one Asterisk by-the-book (for Paralysis), the overview of 'btb' is important: all of the published details are Guidelines, not rules. The ultimate choice always rests with the gaming group. Feel free to assign it an extra asterisk if you wish.
A counter-argument would be that the small (2' long) tentacles, while formidible, can rarely be directed at more than one target... and while that's going on, everyone else can hit it easily (AC7, unlike the AC3 head in AD&D) and its magic avoidance is mediocre at best (saves as Fighter L2).
But at the bottom line, the difference is 25 XP (per asterisk for a 3+1 HD monster), divided among the party... call it 4-5 XP per person. imho, not worth arguing about. So err in the characters' favor.
FM (author of BECMI)
Best Answer
Basic D&D has different Dex modifiers to AC, and different entries for armors worse than chain. The combination can result in up to a 2 point AC difference.
Note that AD&D2E has the same AC's as 1E, but has more armors; I've put the ones not in 1E PH in (parentheses).
For monsters, you won't need to adjust at all. For characters, if AC is 6 or higher, you may want to adjust it.
References:
TSR2010 AD&D1E Player's Handbook, 6th printing, ©1980.
TSR2101 AD&D2E Player's Handbook Electronic Edition (from Core Rules 2.0 CD)
TSR1071 D&D Rules Cyclopedia Scanned/OCR'd Edition (From DTRPG)
GBD1001 Labyrinth Lord (No Art, via DTRPG.)
Edit Note: I added the LL data in for completeness.