[UncleScratchy]

THACO (To Hit Armor Class 0) is the number that must be rolled on a 20 sided computerized die (aka a d20 die that can generate a value between 1 and 20) to be able to successfully hit/attack an Armor Class 0 target. The number rolled on the virtual die is called the "To Hit" number. When someone attacks a target (creature, human, monster) the computer takes the attacker's THACO and subtracts the target's Armor Class (AC) from it. The number derived by this is what must be rolled on the dice for a successful hit. If the number equals or is higher than the require "To Hit" number the attack is successfull and damage is done. Any number lower than that is registered as a complete miss. A roll of a natural 1 is considered a "Critical Miss" and the attacker is penalized by a time delay for its next attack. A roll of a natural 20 is called a "Critical Hit" and the damage from that attack is doubled. Regardless of AC a roll of 1 always misses and a roll of 20 always hits. For example if your char's THACO is 18 and the targets AC is 5 then the computer subtracts the 5 from 18 giving a "To Hit" number of 13. Your char has to roll a 13 or better on the virtual die to do damage to the target. If you look at the various types of armor you see that the better the armor the lower the AC number. So a plate mail wearing enemy (AC 3) instead of leather armor (AC 8) will result in a higher "To Hit" number.

That part is pretty straight forward. Now comes the variables (Modifiers and Magical Items) that alter these basic rules. For example, a char's strength can modify the die roll thus improving the chance to hit and the amount of damage done by melee weapons. Note that Critical hits and misses are "natural" rolls - i.e. the die roll before any modifiers are applied. A STR of 5 modifies the probability of a hit by -2 and if this weakling can hit something the damage is penalized by -1. A STR of 8 to 15, on the other hand, is neutral (no effect) with respect to hit probability and damage modifiers. A STR of 18, however, adds +1 to the probability of a hit and +2 to the damage. For fighters with STR of 18/01-50 the adjustments are +1 and +3 respectively while a STR of 18/51-75 gives adjustments of +2 and +3 respectively and so on. These STR adjustments keep improving up to a max STR level of 25 (godly stats to say the least). In BG1 the highest you can get is 19 STR (with the Tome/Manual) for modifiers of +3 and +7 respectively. Potions or spells can temporarily increase the STR for a limited time improvement. The other variable is Magical Items. A +1 sword for example improves the chance to hit by +1 and if the attack is successful the damage inflicted will be increased by +1. For ranged weapons it is the char's dexterity (instead of STR) that modifies the probability of a hit. DEX does not, however, affect the a amount of damage done.