To @Magick: Question everything. Even the so called experts.
Always try to get a second opinion, without revealing the first, to avoid bias, then confront with first.
Trusting experts is an **appeal to authority **fallacy.
Back to the topic.
I can't say exactly what is now, only Morl can. But I have some understanding how it was 10 years ago.
AC protects you in two ways.
-
If your opponent may fail to pass an armor check and do "miss" zero damage.
-
Your armor reduced X% amount of meele damage based on your AC.
In the Old days, before the sea drank Atlantis, the fail check was an THAC0 roll based on his Hitroll, his Class (THAC0), if he was dual wielding and your AC value. Meele classes like warriors get better chance to not miss than mages like Clerics. Stacking high AC was quite effective to trigger this in the past, and still is at low levels vs mobs. This roll mechanics were very complex.
The armor reduction was a very simple thing like damage reduction % = AC / X , where X is a hard coded value. This is why AC gets better the higher you get, just like saves. Each point is worth more reduction. But good luck squeezing those extra points.
That was in the old days. Now Morl changed stuff. I suspect THAC check has been extremely overhauled, and so have the classes THAC0 values. I think he said that now 800 AC is equals to 600 AC , or something of the sort. Still 600 AC is still a pretty huge damage reduction, probably around 1/3 , from my practical testing less than 1 year ago.
The only thing that players need to take into account is that the higher the AC the better it is. And that if you are missing regularly, them your hitroll is crap.
Some might be interested in knowing the exact values for evaluating the best point in the AC vs HP relationship, but such is to complex and draining to take in account when choosing gear.
Also @Gaunticles, if the AC check comes before of after defenses is moth because the order has no impact. Just like defenses order.
But I think Lloth is right, it checks AC first, then defenses.
Another thing that is worth knowing regarding AC, is that only melee attacks and SOME skills check vs it. For example, throw doesn't appears to check AC for damage reduction.