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?
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.
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.
That is correct, Basic D&D starts off at 9 and AD&D starts off at 10. But the differences are slight. For example no armor is 9 in BD&D and 10 in AD&D, leather is 7 in BD&D vs 8 in AD&D, but chain is 5 in both, and plate is 3 in both.
The difference is inconsequential and represent at most a -1 to hit at the lower armor classes. If you really concerned about it then subtract 1 from AC when it lower than 5 if using an AD&D module. But likely you will be looking up the monster stat in the Labyrinth Lord rulebook anyway. So it is a wash.
That's basically it. Technically the Dexterity adjustments to armour class table is slightly different between 1e and Basic as well, but that's fairly minor - I probably wouldn't worry about that as DM, unless I was helping convert PCs between the two systems.
Note 1st edition also had a more complex system of attack modifiers depending on weapon-vs-armour type that further modified the to-hit rolls for extra realism, but I don't think you'd experience any major problems from not using it.