Pellinore wrote:To
Hit
Armor
Class
Zero (0)
Lower is better.
As in lowest possible DIE ROLL (20-sided) to hit AC=0 (an average fighter wearing plate).
So, a thac0 of 1 will hit AC=0 on all rolls from 1 to 20, thus basically hitting anything non-magical 100% of the time. **edit: I lied. A roll of 1 always misses, even if it's G-d trying to smack down an unarmoured level 1 goblin wielding a bow in close combat for a 4-point AC penalty due to not having a melee weapon equipped**
Thac0s much above 10 will only hit better-armored targets on "natural 20s" (ideal die rolls of 20, before modifiers) aka critical hits, which also land double damage on targets not wearing helmets.
Single and 2-handed weapon style proficiencies (not the weapons themselves, but the style proficiencies at the bottom of the list) will give you criticals on 20s and 19s, thus double the guaranteed hits and double the chances of double damage. Nice, useful, but not 100% necessary - characters can still work well without a single point spent there.
Two-weapon style (dual-wielding, aka putting a weapon in your shield slot) will not double your criticals, but increases your attacks per round by 1 (or by 2, for extra-attack weapons). It looks cool, but only works well with characters who spent proficiency points on dual-wielding. Generally considered only beneficial for damage-dealing after two stars (**) are spent on it, else the penalties are too harsh. For decent fighters like Korgan definitely or maybe even Jaheira, might still give positive results with one star (*) in two-weapon style when facing easy-to-hit opponents, especially if the second weapon is Belm or Kundane (give bonus attacks, which actually count for the harder-and-better hitting main hand, despite what the game says).
Rangers (Minsc, Valygar NPCs, possibly your character should you so choose) get two free ** in dual-wielding at creation, and are thus able to dual-wield effectively right away. Haer Dalis the Blade comes pre-customized for dual-wielding. The other melee combat-class NPCs (Korgan, Anomen, Jaheira, Keldorn, Mazzy) need to be levelled up for a dual-wielding setup, else they will be weaker than an unarmed spellless mage in combat when equipping two weapons at the same time.
If weapons are wielded by a character who isn't actually going to fight with them, like Nalia, Imoen, Jan, Aerie or Viconia, and are simply used for resistances granted, bonus spell slots, castable special effects, etc. - then the dual-wielding penalty really doesn't matter at all.
...Sword and shield proficiency does nothing. I repeat - sword and shield proficiency DOES NOTHING. The ranged protection bonus is laughable. There are exactly *zero* enemy archers in the game capable of hitting anyone equipped with a decent shield, and there's even a shield that returns all arrows to sender automatically - arrowhead first, of course. Don't waste points here, even if you intend to spend some time running around with a shield. Best spent on any other weapon category you might have the slightest chance of ever wielding in the future, even if the likelihood of you switching out your current gear seems 0.01% to you!