Advanced Dungeons & Dragons is the first RPG I played and the first I gamemastered. It is the game system I played the most over the years. It uses a D20 system, before they coined the name, so it’s not what most people associate with a D20 system.
It has a lot of quirks and many sub-systems, various ways to handle similar things and it’s one of things that critics complain about when they talk about AD&D. I don’t think any of us ever played exactly like it’s written. Each of us (GMs or rather DMs) developped our own initiative system for instance.