A small Buddhist image that is placed inside a larger one as an offering. In most cases, older images were placed inside newer ones. The larger image that enshrines the tainaibutsu is called a sayabutsu 鞘仏 (a sheath-buddha), which is most commonly made of wood. Usually, a cavity is cut into the back, the smaller image is placed inside, and then it is sealed up with a panel over the back. Prayers, sutras, Buddhist paintings, and other objects can be dedicated inside the sculpture in addition to small Buddhist images (see *zōnai nōnyūhin 像内納入品).
In regard to materials or types, there is no set relationship between the larger Buddhist image and the smaller one that is enshrined inside it. However, smaller images tend to be made of bronze, while most of the larger ones are wood. The two images may be similar in iconographic type or they may differ. When a temple image was destroyed by fire, in some cases a new image was created, and the remains of the original (sometimes re-carved into a small statue) were enshrined inside the replacement. Sometimes the relationship is not obvious. A donor may request that their *nenjibutsu 念持仏, an image for personal worship, be placed inside an image of a different iconographic type. In any case, the smaller Buddhist image is regarded as the essence of the larger one. The earliest known inserted objects of dedication were not sculptures, but small reliquaries.
Although there are Heian images with small sculptures inside them, the practice gained in popularity after the 13th century, and several examples still exist from the Kamakura period. The image of *Jizō 地蔵 from Denkōji 伝香寺 in Nara, had small images of *Yakushi 薬師 and *Jūichimen Kannon 十一面観音 (Eleven-headed Kannon), made around the same time as the Jizō. A document also found inside the image dates it to 1228. Inside the *Batō Kannon 馬頭観音 (Horse-headed Kannon) from Jōruriji 浄瑠璃寺 in Kyoto, fifty-seven small wooden images of Batō Kannon, along with a *Bishamonten 琵沙門天 figure and fragments of a bodhisattva image were discovered. The image is dated to 1221, and the smaller figures are thought to date from around the same time. Three bronze images of Kannon dating from the 8th century were found inside the Kamakura period *Senju Kannon 千手観音 (Thousand-armed Kannon) from the Jikidō 食堂 (refectory hall) of Kōfukuji 興福寺 in Nara.