Ahh, so you've chosen to read this scroll. It's good to know more of what protects you from potential harm.
Armor absorbs damage equal to the amount of protection it confers (a platemail grants an AC 3, ergo it can absorb up to 7 points of damage). An attacker need only to roll a THAC0 versus the defender's base AC (excluding armor and shield along with any magical bonuses they might give, but including other adjustments that enhance their chances of evading an attack, like DEX adjustment). The defender's armor will absorb damage per point difference from actual THAC0 needed to hit the defender, with AC adjusted by armor.
Hmm.. you don't seem to understand. Here's a example. Kil (THAC0 20) swings a sword at Friss (AC 1 from -4 DEX bonus, hide armor and shield). Kil needs a 14 to hit Friss (base AC=6 from DEX), but Friss' armor would absorb 5 points of damage at 14, 4 at 15, 3 at 16, etc
Only on a roll of 19 and above will bypass Kil bypass Friss' armor.
Critical hits can only be achieved if attack bypasses armor. Spell and magical items that simulate armor (eg. armor spell, rings of protection that create barriers to protect the user) can absorb damage.
This works best with the armor point rule
If you have any questions, comments or anything you'd wish to tell Lord Elmair, send him a at serraqi@pd.jaring.my