Itahi (board monument) is a kind of sekihi (stone monument) mainly used as a memorial tower. It is called itaishi (stone tablet) sotoba (a tall, narrow wooden tablet set up behind a grave for the repose of the dead) or itaishi toba and especially Musashi-style itahi which is typically imaged is called ao-ishi (a generic name for a blue green stone of chlorite schist much used in gardens) toba as it is made by processing pieces of chrolite schist produced in Chichibu.
Itahi is a memorial tower which was used in Buddhism in the Medieval Period. The basic structure is a block of stone processed to a shape of a vertically long tablet on which Bonji (Siddham script) = shuji (Sanskrit seed-syllable in Esoteric Buddhism), the name of a person commemorated, the date of commemoration, the items of commemoration are inscribed. Nijo (double-threaded) lines are inscribed on the head. Actually some portions are omitted.
Itahis are distributed throughout Japan, though mainly in the Kanto region. The dates of their construction are concentrated in fromKamakura period to early Muromachi period. The areas of their distribution are limited to the Honganchi (birthplace of clan) of warriors of Kamakura Bakufu and their territories, and are tightly related to the belief of warriors of Kamakura Bakufu.
They can be categorized into tsuizen (junshu) (a religious service for the repose of the soul of someone) kuyo (to make offerings), gyakushu itahi (a memorial tower built before someone's death), and so on. They are classified into Musashi-style, Shimousa-style, etc. on the basis of shapes, stone material, and distribution areas.
By the way, Musashi-style means something made of bluish stone material called chrolite schist produced in Chichibu/Nagatoro area, but as similar stone material is produced in and around Awa, itahi made of chrolite schist distributed in the Kanto Plain are classified as Musashi-style and those distributed in and around Shikoku are classified as Awa-style. Shimousa-style means itahi made of black mica stone produced mainly at Mt. Tsukuba in Ibaraki prefecture.
They vary in their shapes and in stone material from area to area, and from era to era, and so they have been good objects of research by local historians since before World War II. In the Sengoku period they rapidly died out, and some of existing itahi were disposed or turned into lids of irrigation ditches, etc. They are related to the modern sotoba.