Figuring the To-Hit Number
The first step in making an attack roll is to find the number needed to hit
the target. Subtract the Armor Class of the target from the attacker's THAC0.
(Remember that if the Armor Class is a negative number, you add it to the attacker's THAC0.) The character has to roll the resulting number,
or higher, on 1d20 to hit the target.
Rath has reached 7th level as a fighter. His THAC0 is 14 (found on Table 53), meaning he needs to roll a 14 or better to hit a character or creature of
Armor Class 0. In combat, Rath, attacking an orc wearing chainmail armor (AC 6),
needs to roll an 8 (14-6=8) to hit the orc. An 8 or higher on 1d20 will hit
the orc. If Rath hits, he rolls the appropriate dice (see Table 44) to determine how much damage he inflicts.
The example above is quite simple--in a typical AD&D game combat situation,
THAC0 is modified by weapon bonuses, Strength bonuses, and the like (the next
section “Modifiers to the Attack Roll,” lists the specifics of these modifiers). Figure Strength and weapon
modifiers, subtract the total from the base THAC0, and record this modified THAC0 with
each weapon on the character sheet. Subtract the target's Armor Class from
this modified THAC0 when determining the to-hit number.
Rath is still a 7th-level fighter. He has a Strength of 18/80 (which gives him
a +2 bonus to his attack roll). He fights with a long sword +1. His THAC0 is
14, modified to 12 by his Strength and to 11 by his weapon. If attacking the orc
from the earlier example, Rath would have to roll a 5 or higher on 1d20 in
order to hit (11-6=5). Again, table 44 would tell him how much damage he inflicts with his weapon (this information
should also be written on his character sheet).
The DM may also throw in situational modifiers, (for example, a bonus if the
target is struck from behind, or a penalty if the target is crouching behind a
boulder). If the final, modified die roll on 1d20 is equal to or greater than
the number needed to hit the target, the attack succeeds. If the roll is lower
than that needed, the attack fails.
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