A set of rules used by a game development company for almost every game they publish. It is much cheaper to create the system once, and then tweak it for a specific game than to develop a unique system for each game. It also helps with sales because players will already know the basics, and won't have to learn a whole new set of rules. However, it can backfire if the system doesn't fit the genre they're trying to apply it to.
Most of these end up as Universal Systems, if they didn't start out that way. Contrast House Rules, which the local Dungeonmaster sometimes uses to circumvent System Rules that are not agreeable to them and/or their players.
- The FUDGE System is built to be this.
- Fate began as a fully generic system based on FUDGE in its first and second editions; its third edition, however, was the basis of several games beginning with Spirit Of The Century and was only this trope for several years. Finally, with the publication of Fate Core, it's back to being a true Universal System. (It's also no longer really based on FUDGE; it uses a few FUDG-y concepts and FUDGE dice, but is rewritten and rebuilt from the ground up, partially so that it could shed the OGL for an even more permissive Creative Commons license.)
- The d20 System by Wizards of the Coast.
- 2d20 system by Modiphius Entertainment.
- The Palladium or "Megaversal" system by Palladium Books.
- The d10 system of Fantasy Flight Games' Warhammer Fantasy Roleplay.
- The Storyteller system by White Wolf.
- Followed by their Storytelling system, where each game refers back to the core system in a single book (World of Darkness).
- The D6 system by West End Games.
- The Unisystem by Eden Games.
- The Tri-Stat System by Guardians Of Order
- The Basic Roleplaying system by Chaosium is an interesting case: Chaosium mutated the original RuneQuest into a half-dozen systems with the same core mechanics (percentile skills and the same seven stats, more or less), and collected most of the variations into one system with numerous options.
- Except for Pendragon, which changed too radically.
- Mongoose's version of RuneQuest is just different enough from Chaosium's system to avoid lawsuits, but thanks to the first edition's Open Game License became the basis for other variations, mostly fan-made.
- The D10-based system used by AEG for Legend of the Five Rings and 7th Sea. Both used the same basic die rolling mechanic, but were otherwise quite different.
- The Hero System is a textbook example of a single game (Champions) evolving into a House System (as used in the original editions of Star Hero, Fantasy Hero, Justice, Inc., Danger International, and Robot Warriors), and then back into a single Universal System game. Most of the older side-games were revived during the 5th edition era as genre-advice sourcebooks.
- GURPS, kinda. Steve Jackson Games does publish other systems, Toon being the best known.