I've been running 1e for 25-odd years and still haven't figured out THAC0. To me, it just adds an unnecessary layer of complication - I don't care whether a swing hits AC 0, I care whether it hits the specific AC of the particular opponent at the time - all I want to know is whether the base roll, bonuses, penalties, fight level, and AC end up adding to 21 or more. THAC0 doesn't help me there, and instances of both the target's AC being 0 and the attacker's listed THAC0 rating being accurate for the situation happen rarely enough that I'm not going to waste the time checking.
Lanefan
THAC0 just converts all the tables into a simple formula to create a number line
-1 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10
THAC0 is at the zero on this line.
say THAC0 12
put down the number 12 under or over the zero on the character sheet. Forget which one has the THAC0 line on it, but you can make it yourself.
Then write down the numbers along the rest of the line. It is elementary school math.
AC 1 moves one place along the line so it changes THAC0 by one. gives you one better to hit that AC with. Instead of needing 12, you only need 11 to hit AC 1.
AC basically is your modifier to hit when comparing to AC0.
With the line you dont need to tell the DM anything or him to do any math.
You roll a 10 with THAC0 12, you can hit AC 2.
Got weapon or other bonus, then add it to the dice roll. Got a penatly, then take it away from the dice roll.
+4 weapon makes your roll a 14. looking at the number line then you 10 became a 14 so you could hit an AC -2.
Same as BAB, just with THAC0 there is a greater chance to hit things in the long run wherein BAB allows for ACs that you could never hit.
All math is just addition.
Addition = ....addition
Subtraction = adding the negative
Multiplication = adding something to its self a number of times
Division = adding a part rather than a whole of something to something else.
The reason you want to know what it takes to hit AC0 is for the mid range of the entire AC range.
BAB makes certain ACs useless. Why start at 10? Why not start at AC0?
Because you would have things that could never be missed unless you impose a critical fumble rule. I don't care who you are, I have seen all kinds of people swing and ax and MISS a tree.
At least THAC0 doesn't waste parts of its range, and is a simple formula.
DM uses:
THAC0 - AC = # needed to hit (dice roll + modifiers)
Player uses:
THAC0 - (Number rolled + modifiers) = AC you can hit
pick one.
using the player one and the previous THAC0 of 12 and the +4 weapon/whatever bonus.
12 - (10+4) = -2
12 - 14 = -2
yup. it works
using the one for DMs
12 -(-2) = 14
12 + 2 = 14
With a THAC0 of 12 you need a 14 to hit a AC -2 whatever.
So if you dont know the AC it doesnt matter. THAC0 works for both player and DM. It is just most commonly expressed form the DM perspective where he known the AC needed rather than given what the players would need to tell the DM.
For all that care, not just you that is.
And as previously mentioned, sooner or later you will learn the AC no matter which method you use. BAB or one of the THAC0 formulas. Then you can just use the DM formula to figure out what you need to roll, and what to attack with.