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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?

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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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  • \$\begingroup\$ It is also worth noting that the 1e AD&D Monster Manual, which was the first AD&D book published, still used the original/basic ACs. \$\endgroup\$ Jun 4 '13 at 13:56
  • \$\begingroup\$ Actually, @RobertFisher , 1E doesn't. \$\endgroup\$
    – aramis
    Aug 29 '14 at 7:45
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    \$\begingroup\$ See odd74.proboards.com/thread/2515 for some discussion about this. The 1e MM also only used the 5-way alignment system instead of the eventual 9-way system of the PHB. \$\endgroup\$ Aug 29 '14 at 14:47
  • \$\begingroup\$ FWIW, Bronze Plate, Field Plate and Full Plate were introduced in the 1st ed. Unearthed Arcana (or earlier). \$\endgroup\$
    – ucbpaladin
    Nov 18 '19 at 21:08
  • \$\begingroup\$ @RobertFisher 5 way vs 9 way makes no real difference in play. \$\endgroup\$
    – aramis
    Nov 21 '19 at 19:13
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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.

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  • \$\begingroup\$ I concur...you're looking at a 5% difference(at most..depending on armor type) in "To-Hit" values. Also some old school modules list creatures as Orcs(3): HPs (3, 3, 5) and don't even list AC, which helps put players/creatures on the same AC page, assuming the same reference material for characters/creatures. \$\endgroup\$ Aug 20 '10 at 15:06
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    \$\begingroup\$ otoh some players are rather insistent about that 5%. \$\endgroup\$
    – ExTSR
    Aug 21 '10 at 1:52
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    \$\begingroup\$ And, due to the different tables for both armor and dex mods, it's up to 2 points different. \$\endgroup\$
    – aramis
    Dec 13 '10 at 6:50
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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.

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