Funmunitions was recently asked: “What house rules do you use in any of your 1st Edition AD&D games?” (we actually DO still play those from time to time, but usually only as one-shot mini-campaigns). We provided the following SNIPPET of the rules we have recently tried..
 Adding a d100 (%) mechanic for perception – all unless noted are +d6% per level over 1
Visual: d10% + 25% + [1/2 of Int score]
Audio: d10% + 25% + [1/2 of Wis score] + [1/2 of hear noise %]
Tactile: d10% + 25% + [1/2 of Dex score]
Olfactory: (taste and smell): d10% +25% + [1/2 of Charisma]
Supernatural: -5% + d10% +1% per level: “Is this place haunted?”
Magical: d10% + 25% + [1/2 of Wis score]: “was that a spell that targeted me?”
Mental: d10% + 25% + [1/2 of Int score]: “Can I remember that?”
Skills: d10% + [1/2 of Int] + d4% per level: “Do I know how to do that thing that isn’t a proficiency?”
Generally, if the DM asks for a perception d100 check, the check is made at a “passive” -15% (+1% better each level over 1). If the player asks for the check it is rolled normally.
 Personality traits – we required 5 good ones and 5 negative ones at level 1, with +1 more of each chosen each level. These might be things like “easily frustrated”, “brave”, etc. chosen by the players. If there were well-role-played the DM would usually add minor game mechanics to these, like giving the “brave” character a +1 to fear saving throws.
Some of these traits made their way into this background generator: d1000 Neutral Quarks
 Mainly due to our exposure to Palladium system and 3.5 D&D, we adopted a “defense roll” instead of just having a static AC. Thus, if a player with an AC of 4 were being attacked, they would also make a d20 roll, adding (in their case) a +6. A character with an AC of 10 would add +0 to their defense roll, etc. If the attack roll ties the defense roll, the attack hits but causes half damage.
 everyone was dealt 5 random Magic the Gathering* cards – they used this to build their character backgrounds.
 I added an extra stat: Luck. This is incredibly useful for those moments where the rules run out, when a monster needs to decide who to attack (the PC who fails their luck check by the most), when some amount of treasure needs to be determined, etc. The Luck check is on d20 trying to roll the value or under (much like 2nd edition D&D).
 Armor soaks damage: take the AC value the armor provides, divide by 2 and round down – this is how much damage the armor soaks when hit by a weapon. Thus, chain main (AC 5) would soak 2 damage per hit, chainmail +1 would soak 3, etc. We experimented with limitations to this as follows:
- The damage soak effect only happens X% of the time, where X = the AC value of the armor x 5%. Thus, chainmail (AC 5) only soaks damage 5 x 5 = 25% chance.
- The armor has a set of hit points that are lost when damage soak happens; assume a piece of armor has 10 hit points per AC point it provides: using the chainmail example a suit would have 50 hp. When the armor’s HP was depleted, the AC value of the armor would drop 1 and its hp would reset, until it was destroyed when its AC became 10. MAgic armor only looses hp when hit by magic weapons.
 Critical hits – on a natural 20 that hits, when a 20 is not needed to hit, a critical hit is scored X% of the time, where X = the AC of the armor worn x 5%. Thus, a suit of chainmail providing AC 5 and a shield (AC 4 then) would suffer a critical hit 20% chance on a 20. The minimum chance is 5%, with magic armor or weapons reducing or increasing the chance of a critical hit by +10% per plus. We used the old Dragon magazine’s crit tables (“Good Hits and Bad Misses”) – I will try to attach them –