This is a post about THAC0.
This system for determining whether or not a player character hits in 2E hinges on two factors: THAC0 and Armor Class.
THAC0 is an acronym for To Hit Armor Class Zero. A character's THAC0 value is the number he must roll on a d20 in order to hit a thing that has an Armor Class of zero. The value of the target's Armor Class is applied to the attack roll of his aggressor.
Attack roll +/- target's armor class +/- any other modifiers = Modified Attack Roll
If Modified Attack roll is equal to or greater than THAC0, the hit succeeds.
Complex in its simplicity, no?
You may be thinking, "Fuck that! Why would I want to do that in my head every time I roll the dice?"
Good news, gentle player, you don't. Such things are for the DM to compute. As a player, you tell the DM what you've rolled (plus any modifiers) and your THAC0. The DM adds in the target's armor class and any other modifiers he may have and lets you know if you hit.
The magic of this is that the player is only acutely aware of improvement through examining results. As opposed to an "active" value, like a bonus or penalty, THAC0 is passive. Every time a player attacks, he is not adding his THAC0 or evenly actively considering it unless the DM asks him for it. If the DM has prepared a reference sheet in advance, he might never ask for it.
It should become apparent, however, to the player that their character is hitting more often as their THAC0 goes down. But because they are not reminded with every swing of their mace that they have incrementally improved, they perceive less of the hard mechanics of the game.
But wait! Isn't the to-hit system in 3E the exact fucking same? You tell the DM what you rolled and he tells you if you hit or not?
Yes and No.
In a general, yes. That's how most games work. The players do a thing and then the DM does his things and tells the players about it. Repeat.
In 3E, that Base Attack Bonus is going up. If not this level, then next level. The player must be constantly aware of the arbitrary rule that represents his improvements in combat. Some feats (I'll do a whole other post on why I hate feats) even use those bonus as bargaining chips and let you redistribute them. (I'm looking at you, power attack.)
The bonus is at the forefront. With THAC0 it is in the background. Almost invisible.
Try it out sometime.
To convert, take the sum of a character's arbitrary to-hit bonuses. Do not factor in strength bonuses, bonuses because of fighting style or particular skill with a particular weapon, only the bonus you receive to your hit roll because you are an X-level Adventurer.
Got the sum total? Great. Subtract it from 20 - There's your THAC0.
For armor class, determine how much better your armor is than no armor. Is it two points better? Is it eleven points better? Whatever. Figure out how much better your armor is than no armor according to your system. Then subtract that from 10. That is your new armor class. Have your DM do likewise for all his shit.
Or just pickup (read "download") a copy of the 2E rules. You know, whatever.