Huáisù 懷素 (624–697) was an early-Tang Vinaya master and a leading figure of the Dōngtǎ 東塔 (Eastern Pagoda) school of Sìfēn Vinaya, one of the three major sub-traditions of the Tang Sìfēn lǜ exegetical tradition (the others being Dàoxuān’s 道宣 Nánshān 南山 and the Xiāngbù 相部 of Fǎlì 法礪). A native of Jīngzhào 京兆 prefecture, he became a disciple of Xuánzàng 玄奘 (玄奘) and assisted in the latter’s translation projects before turning to Vinaya specialization under Dàoxuān himself. The principal extant works of Huáisù are the Sēng jiémó 僧羯磨 (KR6k0047, T40n1809) and the Ní jiémó 尼羯磨 (KR6k0048, T40n1810) — the most influential Sìfēnlǜ karmavācanā compendia for monks and nuns respectively, used continuously throughout Chinese, Korean, Japanese, and Vietnamese Buddhist history. He is to be distinguished from his more famous namesake Huáisù 懷素 (725–785), the Tang calligrapher of the “Wild Cursive” (kuángcǎo 狂草) style, who was a different person.