To make an attack in 2e AD&D, you subtract the enemy's AC from your THAC0 score before rolling. So if your THAC0 is 15 and the enemy's AC is -1, you subtract -1 from 15, meaning you need a 16+ to hit. While it works fine, many people find the system unintuitive since you are really adding the inverse of the AC to your THAC0.
Why not simply use the defender's AC as a modifier to the dice roll itself? No need to recalculate your THAC0 that way, and you don't use any inverse values, so to speak. Mathematically it's the same result, but sees to me to be more intuitive. If the DM doesn't want to reveal the enemy's AC, the player can simply roll the dice and compare it to his THAC0, giving a number indicating what AC has been hit. For example, if the player with the 15 THAC0 rolls a 17, he announces he has hit AC 2 or better.
Yes? No? Maybe?
Why not simply use the defender's AC as a modifier to the dice roll itself? No need to recalculate your THAC0 that way, and you don't use any inverse values, so to speak. Mathematically it's the same result, but sees to me to be more intuitive. If the DM doesn't want to reveal the enemy's AC, the player can simply roll the dice and compare it to his THAC0, giving a number indicating what AC has been hit. For example, if the player with the 15 THAC0 rolls a 17, he announces he has hit AC 2 or better.
Yes? No? Maybe?