An astronomically-inspired taxonomy that breaks RPGs into 3 waves: Alpha Prime (D&D and its immediate descendants, including games as recent as the 1986 edition of Warhammer Fantasy Roleplay), Beta Prime (supposedly 1990s games, but fully half are actually from the 1980s), and Gamm Prime (indie games, which includes Forge games as well as Robin D. Laws). OGL and OSR are listed separately as "satellite worlds and moons".