[RPG] AC differences between AD&D 1E and Basic D&D

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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?

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).

_____________________ Armor for a given AC _______________________
AC  Basic           Advanced D&D 1E (2E)             Labyrinth Lord
10  --- . . . . . . None . . . . . . . . . . . . . . ---
 9  No Armor  . . . Shield Only  . . . . . . . . . . Unarmored 
 8  Shield Only . . Leather/Padded . . . . . . . . . Padded, Leather
 7  Leather . . . . Studded Leather, Ring Mail . . . Studded Leather
 6  Scale Mail  . . Scale, (Brigandine, Hide)  . . . Scale Mail
 5  Chain Mail  . . Chain Mail . . . . . . . . . . . Chain Mail
 4  Banded Mail . . Banded, Splint, (Bronze Plate) . Banded,Splint
 3  Plate Mail  . . Plate Mail . . . . . . . . . . . Plate Mail
 2  --- . . . . . . (Field Plate)  . . . . . . . . . ---
 1  --- . . . . . . (Full Plate) . . . . . . . . . . ---
 0  Suit Armor  . . ---  . . . . . . . . . . . . . . ---

AC Dex Adjustment
Dex:             3  4  5  6  7-8 9-12 13-14 15  16  17 18-19 20-21 22-23
Basic D&D:      +3 +2 +2 +1  +1   +0   -1   -1  -2  -2   -3   -4    -5 
Advanced D&D 1: +4 +3 +2 +1  +0   +0   +0   -1  -2  -3   -4   --    --
Advanced D&D 2: +4 +3 +2 +1  +0   +0   +0   -1  -2  -3   -4   -5    -6
Labyrinth Ld:   +3 +2 +2 +1  +1   +0   -1   -1  -2  -2   -3   --    --

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.

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