天台所說。地獄，餓鬼，畜生，修羅，人間，天上，聲聞，緣覺，菩薩，佛十界中之各界皆具有十界，故以十界乘十界而為百界也。
The ten realms each of ten divisions, so called by the Tiantai school, i. e. of hells, ghosts, animals, asuras, men,devas,śrāvakas,pratyekabuddhas, bodhisattvas, and Buddhas. Each of the hundred has ten qualities, making in all 百界千如 the thousand qualities of the hundred realms; this 1, 000 being multiplied by the three of past, present, future, there are 3, 000; to behold these 3, 000 in an instant is called 一念三千 (一念三千之觀法) and the sphere envisaged is the 百界千如.
一百界 四聖六凡的十法界，一一界中，又具十界，總稱「百界」。（參閱「百法界」3128、「百界千如」3128）p3128
百界總目錄 地獄、畜生、餓鬼、阿修羅、人、天、聲聞、緣覺、菩薩、佛之十界，界各互具十界，則相乘而為百界。
地獄、餓鬼、畜生、修羅、人間、天上、聲聞、緣覺、菩薩、佛等十界，每界各具十界，乘之便是百界。