One of the things I love about the simplicity of older versions of D&D (and their retro-clone equivalents) is that it is an open invitation to create house rules. Whereas this freedom is increasingly limited as the system becomes more comprehensive in later editions, OD&D actually offers up an example of a house rule — the now familiar d20 system for combat. Originally designed to be used for Chainmail, OD&D invited players to come up with their own rules to deal with in-game situations (like how to do combat when you don't own Chainmail). Recently, James over at Grognardia meditated upon the AC adjustment for individual weapons that can be found in 1eAD&D. Specifically, he invited us to understand AC as a class, instead of as a target number. This got my inner geek excited, because armor doesn't prevent you from being hit, rather it prevents you from taking damage. Allowing individual weapons to be more or less effective depending upon armor worn is a simple way to represent this.
Thus, I have taken the invitation of OD&D and of James and come up with my own house rule. Two problems arise out of the system presented in 1eAD&D that make it too cumbersome — too many weapons and too many armor classes. That latter is taken care of if you differentiate armor and a defensive (dodge) bonus. Dexterity, shields, magic and cover all make you harder to hit, therefore affect the defensive bonus. Once the shield is understood as a defensive bonus, that leaves only 4 armor classes in OD&D — platemail (3), chainmail (5), leather (7), and none (9). For purposes of this explaination I will designate them ACI-IV with ACI being platemail. These will provide a target number to do damage (NOT to be hit!) depending on what type of weapon is being used.
Comparing the AC adjustments for these four armor classes in 1eAD&D results in five discernable patterns in meleee weapons and two patterns in ranged weapns. Thus, there are five melee weapon classes (MC) and two ranged weapon classes (RC). The following table gives the target number to do damage against each armor class with each weapon class:
|ACI||ACII||ACI II||ACI V|
ACI = Platmail
ACII = Chainmail
ACIII = Leather
ACIV = None
Shields do not affect Armor Class, rather they modify the Defensive Bonus.
MCI = Club, Dagger, Staff, Unarmed, Improvised
MCII = Flail, Mace, Military Pick, Morning Star, Warhammer
MCIII = Sword, Axe
MCIV = Spear, Javelin, Trident, Polearm
MCIV = Halberd, Lance (Charge), TH Sword
RCI = Short Bow, Sling, Thrown Wpns
RCII = Cross Bow, Long Bow, Machine (Catapult, etc.)
A player rolls a d20, adds an attack bonus based on Character Level, Strength bonus (for melee), Dexterity bonus (for ranged), and magic, and subtracts the target's defensive bonus (shield, Dexterity bouns, magic, and cover). If the result is the target number or higher, damage is done.
Attack bonuses for Character Level can be determined by either of the following tables (the first spreads bounses out over each level in a 3e fashion based on 1eAD&D to hit tables, the second more closely follows the to hit tables in older editions of D&D):
The primary problem created by this system is monsters — what do you use for their armor class, their weapon class, their attack bonuses, and their defensive bonuses? A monster's armor class requires some math. Decide which class best represents the hide of the creature (i.e. leather for mammals, chain for reptiles, plate for insects or dragons). Take the base AC of the monster write-up, take the difference from 3 for plate, 5 for chain, 7 for leather, or 9 for none and use the result as a defensive bonus. For example, a Dragon with an AC of 2 would use ACI and have a defensive bonus of 1 (3 — 2 or 17 — 16). In another example, a Giant Fire Beetle has an AC of 4. If we determine that the insect carapace is like platemail, the difference is actually a defensive penalty of 1. If you aren't comfortable with a penalty, you can always choose to understand the carapace as more like leather or chaimail and have defensive bonuses of 3 or 1 respectively.
Weapon Classes are easier — just choose a class that best simulates the type of natural attack. For example, claws would act like swords, a rocky fist like a mace, and spikes like a spear. A manitcore's tailspikes would be like thrown weapons and a dragon's breath weapon would be like a catapault.
To determine the attack bonus of a monster, simply use their HD as their bonus with a max bonus of +15. HD less than one receive no bonus. For creatures with HD+X, such as HD 1+1, you can choose whether not to have them fight at one HD higher.