[daan]

Now you allready now how THAC0 works, we'll get to the AC.
Where THAC0 determines wether you hit an enemy or not,
Your AC determins wether an enemy hits YOU.

The lower the better, with a BG-max. of -20 (not sure)
It works exactly as in the hobgoblin-example, you deduct it from the enemy's THAC0, and the result is a number which your enemy must minimally roll to hit you.

When i say roll, i mean that the computer generates a number randomly,
i call it roll, because it used to be done by dices. This is also where the words "hitdice", 2d4 1d8 etc. come frome.
2d4 means 2 dices of 1 to 4.
(The computer will subsequently generate two numbers of 1 to 4)

a hitdice is a dice that determines the HP of your enemy. A hitdice has eight sides. For example :
an Umberhulk has 8 hitdice, this means an Umberhulk can have upto
64 hp. His HP-scala ranges from 8 to 64 ( 8x1 to 8x8 )

The mage-spell "Death-spell" kills any creature of 8 hitdice and lower, thus killing any creature that has 64 hp or lower.
Note though, that this spell looks ate the MAX.hp .. NOT at the CURRENT, a 40th lvl fighter with 2 hp left, WILL survive a deathspell, since his MAX. hp is 300 or something.

To wrap THAC0 and AC up:
to see if you can hit an enemy / an enemy can hit you:
DEDUCT his AC from your THACO.
That number you must minimally roll to hit him.

example:

THAC0 = 15 AC = -1
Number you must minimally roll to hit the enemy is :
15 - - 1 = 15 + 1 = 16

( - - becomes + ... a nice way to illustrate this is to say : " Im not, not going to the party, means you ARE going to the party)

THAC0 = -1 AC = 5
to hit number = -1 - 5 = -6

(meaning you will ALLWAYs hit, except if you roll a natural 1)