Yags is a generic rules system that can be adapted to many different genres and settings. The core rules are designed for a Medieval European setting.

By default, Yags is designed for low fantasy - PC's may be heroic, but even a hero can be killed with a single sword blow, or a well placed arrow. However, much of the combat system has been designed from the point of view of films such as Flying Daggers, Hero or Kill Bill - a hero can achieve feats normal people can't, but they are not invulnerable to normal people. In this sense, it is more like GURPS than d20.

Everyone has a set of attributes, of which there are eight primary ones - strength, stamina, agility, dexterity, perception, intelligence, empathy and will. A typical adult human has a score of 3 for all attributes. Scores above 5 are truly heroic, and scores less than 2 are typically crippling. All beginning characters will have attributes in the range of 2 - 5.

Everyone has a number of skills as well, which range from zero upwards - 15 is about the maximum, though scores of 16-20 (or higher) may be had by truly exceptional beings. A score of 4 is considered to be a typical professional level, and 10 or more is considered very exceptional, and limited to named characters.

In order to make a skill check, the player rolls a d20, and adds their attribute multiplied by their skill. This result is compared against a target difficulty, and if the target is equalled or exceeded, then the attempt was a success.