Lit. "the exposition of the Dharma mudra." A mudra or hand gesture *in 印 symbolizing the preaching by the Buddha. It is formed by holding the right hand with the palm turned outwards in front of the chest in the manner of the *semui-in 施無畏印 and joining the thumb and forefinger (e.g., *Yakushi 薬師 enshrined in Yakushiji Kondō 薬師寺金堂 in Nara) or the thumb and middle finger (e.g., *Rushanabutsu 盧舎那仏 enshrined in Tōshōdaiji *Kondō 唐招提寺金堂 in Nara).
In the iconography of Indo-Tibetan Buddhism, the mudra formed by joining the thumb and forefinger of the right hand is known by various names such as vitarka-mudra (mudra of argumentation), karana-mudra (mudra of (religious) action) and vyakhyana-mudra (mudra of exposition), and the Sino-Japanese term seppō-in is probably a translation of vyakhyana-mudra. There are some Japanese scholars who refer to the *tenbōrin-in 転法輪印 (Sk: dharmacakra-mudra) as seppō-in, while others regard the dharmacakra-mudra and vyakhyana-mudra as being identical.
According to various interpretations found in Japan (e.g., Gohō 杲宝 [1306-62], Dainichikyōsho ennōshō 大日経疏演奥鈔), the seppō-in formed by joining the thumb and forefinger can symbolize the Dharma-body, hosshin 法身 (Sk: dharmakaya). Joining the thumb and middlefinger can symbolize the recompense-body, hōjin 報身 (Sk: sambhoga-kaya). Joining the thumb and third finger can symbolize the transformation-body, ōjin 応身 (Sk: nirmana-kaya). No similar interpretation is, however, to be found elsewhere in Asia. Because of the ambiguities associated with the term seppō-in, it has become general practice in the study of Buddhist art in present-day Japan to use concrete descriptions, such as thumb and forefinger joined, when referring to this mudra.