I'm not an "old-school renaissance" kind of person, but I'm running a D&D game that is supposed to use ONLY the 1981 Moldvay rules - not because I'm a purist, but because that's the oldest version of D&D I have.
I'm really enjoying reading through the rulebook. It's neat how focused and "game-like" the rules are. This happens in a dungeon! Here are the eight possible reasons why the party went in there! And so on.
Here, however, is a particularly crazy rule that I wanted to share:
Each alignment has a secret language of passwords, hand signals, and other body motions. Player characters and intelligent monsters will always know their alignment languages. They will also recognize when another alignment language is being spoken, but will not understand it. Alignment languages are not written down, nor may they be learned unless the character changes alignment. When this happens, the character forgets the old alignment language and starts using the new one immediately.
This made perfect sense to me when I was 13.