The three sections, divisions, or periods of Buddha's teaching in his life- time, known as 序分, i.e. the 華嚴, 阿含, 方等, and 般若sūtras; 正宗分, i.e. 無量義, 法華, and 普賢觀sūtras; and 流通分, i.e. the 涅槃經; they are known as introductory, main discourse, and final application. There are other definitions.