Universal (Common Set of Rules)

 GURPS the Generic Universal RPG System

The Universal in GURPS stands for one core set of rules. This means the GM and players only need to learn one set of rules. Supplements may add additional skills, advantages, and certainly gear but each uses the same standard mechanics and rules.  The GM is giving optional rules, advanced combat, various magic systems, advanced social rules, etc.  These are optional and modular though, meaning the core rules remain the same with various modules that can be easily added expanding on the basic framework.

This makes the learning curve when changing settings, genres, or even styles of play much easier. In fact when optional rules are added players can often ignore them if they choose. For example a player can stick with the basic combat rules even in a game that adds the optional and detailed combat rules.

Combat Primer

Skill Primer