well, I started playing when THAC0 no longer applied, but as far as the D20 BAB system for D&D goes, AC is calculated 10 + Armor Bonus + Shield Bonus + Dex Modifier + Any misc./magical modifiers (that's where rings, gauntlets, and other talismans go.). Mismatched armor parts like bracers, gauntlets, and other single parts usually add up for +1 to AC, unless they're magical (bracers of armor, for example, range from +1 to +12, I think.). How long has it been since you've played?