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Message: <blockquote data-quote="T. Foster" data-source="post: 3451704" data-attributes="member: 16574">The original set has a chart (two charts, actually -- one for men attacking (based on levels) and one for monsters attacking (based on hit dice)) with 8 armor classes, numbered 9 (worst = no armor) to 2 (best = plate & shield). The tables are arranged so that hitting an AC one step better requires a roll 1 higher on a d20, so that a 1st level character who requires a 10+ on 1d20 to hit an AC 9 opponent requires an 11+ to hit an AC 8 opponent, down to 17+ to hit an AC 2 opponent. However, no particular mention of this mathematical relationship appears anywhere in the rules. Magic armor (which is always magic plate mail, btw; there were no suits of magic leather armor or chain-mail until AD&D) and shields do not decrease the character's AC; rather they subtract from the attacker's die roll -- so a character wearing leather armor and a +2 shield is still AC 6 (leather & shield) but his opponents must make their attack rolls at -2. A character in +2 armor with a +3 shield (the best possible in the original rules) is AC 2, but opponents' attacks are made at -5. There is an ambiguous reference in the rules that suggests that if a character doesn't have magic armor, a magic shield only adds its bonus 1/3 of the time.Supplement I changed this system, making it so that magic shields and armor (and Dexterity bonus, which was introduced in this Supplement) does not affect the attackers' die rolls but rather changes the character's AC. So a character with leather armor, +2 shield, and Dex 16 (+2 defense bonus) is AC 2 (base AC 6 +2 for magic shield +2 for Dex). For this reason, the AC scale was also extended below 2, so that, for instance, a character with +5 armor and +5 shield (the best possible in Supplement I) would have AC -8 (base AC 2 +5 for magic armor +5 for magic shield). To determine the die roll needed to hit AC -8, a chart instructs the attacker to add 10 to his score required to hit AC 2 (so if you're a 13th level fighter who requires an 8+ to hit AC 2, you'd need an 18+ to hit AC -8. Although the possibility of doing so is never mentioned anywhere in the OD&D rules, clever fans soon realized that if you extrapolate the to hit table two places to determine the character's score required to hit AC 0 (i.e. the score required to hit AC 2 +2 per the chart in Supplement I) that that value can be used in a formula to take the place of the tables: THAC0 - target AC = die roll required to hit (also, die roll required for hit + target AC = THAC0).So, to summarize all that, if you're already familiar with THAC0, that's the same system found in OD&D, but it's never described in those terms -- the assumption is that you'll use the tables.</blockquote>

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