to hit Armor Class zero

Herein I summarize the use of THAC0 & several alternatives.

THAC0 first saw print (perhaps) with the AD&D DMG. It wasn't an official technique until AD&D2e.

THAC0 was also explained in the RC c. 1991. That would have been roughly the same time as 2e. Perhaps it was in the Companion, Master, or Immortal set? Also, some modules explained THAC0.

I also discovered that Mayfair's Fantastic Treasures (c. 1984) includes THAC0.

An article (by Phil Alexander?) in Underworld Oracle issue 3 (date?) described the THAC0 technique, although it doesn't actually use the term. The UO article cites a Trollcrusher article by Gary Brittain as the origin. It then goes on to expand on the technique.

Using the table instead

Many people find the "to hit" charts (especially in Classic D&D) quick & easy enough. No need for THAC0 or math. Players can copy their line off the table onto their character record sheet/card.

The one disadvantage is that you have to have the table in front of you.

Standard THAC0 use

Subtract the "to hit" roll plus modifiers from THAC0 to determine the best AC hit. Or, subtract the target's AC from THAC0 to determine the target number that the "to hit" roll plus modifiers must exceed.

• THAC0 - (roll + mods) ≤ AC
• THAC0 - AC ≤ roll + mods

The first method can be used by players. The second method can generally be used by DMs only, since the players may not know the monsters' AC.

Consider a 1st level character (THAC0 19) attacking an AC 4 monster:

• THAC0 19 - roll 15 = AC 4 or more
• THAC0 19 - AC 4 = roll 15 or more

Note that, while THAC0 broke down when it met the repeating 20s rule on the 1st edition AD&D combat tables, 1981 D&D does not have that rule. (The 20s are repeated on the 1981 D&D combat charts only because a 20 was an automatic hit.)

Using THAC0 "backwards"

Another way for a DM to use THAC0 is to add the target's AC to the "to hit" roll plus modifiers. If this is greater than or equal to the attacker's THAC0, the attack hits. The nice thing about this is that it uses (barring "to hit" penalties & negative ACs) only addition instead of subtraction.

• AC + roll + mods ≥ THAC0

Considering the 1st level character (THAC0 19) attacking an AC 4 monster example this way:

• AC 4 + roll 15 = THAC0 19 or less

Over 20

Another way to eliminate subtraction is to precalculate an attack bonus (AB) to take the place of THAC0. The attacker's AB & target's AC are added to the "to hit" roll & modifiers. If the total is 20 or more, the attack hits.

• 20 - THAC0 = AB
• roll + AB + mods + AC ≥ 20

In the example case:

• 20 - THAC0 19 = AB +1
• roll 15 + AB 1 + AC 4 = 20

Roll low

This method also depends upon a precalculated AB. The AB & modifiers are applied to the target's AC. The "to hit" roll must then be less than or equal to the target number.

• AB = 21 - THAC0
• AC + AB + mods > roll

e.g.

• AC 4 + AB 2 = roll 6 or less

Full on 3e style

The downside to this method is that you have to precalculate two numbers: attack bonus (from THAC0) & "new AC" (NAC) (from AC). This is fine for PCs, but a pain for a DM using a module that already lists THAC0 & AC.

• AB = 20 - THAC0
• NAC = 20 - AC
• roll + mods + AB ≥ NAC

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Classic D&D

last updated 2 years ago #