The three realms of samsa^ra: （1） The desire realm 欲界， where one is preoccupied by desires for physical gratification. （2） The form realm 色界， where one is free from the desires for physical gratification and experiences subtle form. The is the locus of the four meditation heavens 四禅天. （3） The formless realm 无色界， the highest realm of samsa^ra， where one is free from material existence. This is the locus of the practice of the Four Formless Concentrations 四无色定.（muller）
According to Vasubandhu's Abhidharmako/sa （T1558.29.40c-67a）， the three realms are broken down into various realms and heavens. The Desire Realm （欲界 yùjiè） consists of twenty places. They are the four continents （四大洲 sìdàzho^u）， the eight burning hells 八热地狱， and the six heavens of the desire realm 六欲天. Beyond the sixth desire realm heaven are the seventeen dhyana heavens of the form realm 色界十七天. Above the form realm heavens is the formless realm （wusejie 无色界）， where there is no material existence. There are four abodes （awarenesses， meditations， states） in the formless realm 四无色.