#### weasel fierce

##### Villager

So lets deconstruct the math and see how it works.

I'll use AD&D to refer to the pre-D20 editions (as they all works the same, hit roll wise) and 3.x to refer to the D20 editions.

Finding out what you need to roll equal to, or better:

3.x: Deduct Base attack bonus from armour class. Result is number you roll equal to or better to hit.

AD&D: Deduct armour class from THAC0. Result is number you roll equal to or better to hit.

(Exactly the same)

Finding out what armour class you hit with the number you just rolled:

3.x: Add the rolled, modified number to your base attack bonus

AD&D: Deduct the rolled, modified number, from your THAC0.

(the same, except one is a deduction instead of a plus)

Basic to hit roll mechanism:

3.x: Roll 1D20. Add base attack bonus. Equal or better than armour class is a hit.

AD&D: Roll 1D20. Add targets armour class. Equal or better than THAC0 is a hit

(exactly the same)

Looking up numbers on a chart ? Both versions use two single numbers to determine attack success, both of which would be written on your character sheet, and unchanging till next level (Thac0 / BAB and armour class)

Then there's armour class.

3.x AC: Number shows how hard it is to hit you. Thus, a high number is good.

AD&D AC: Number shows how easy it is to hit you. Thus, a low number is good.

Saving throws are the other thing, but I'll leave that for another day.

Mind, this isnt an edition war attempt but a basic deconstruction showing that THAC0 works almost exactly identical to the D20 system.