Among the five schools of the , Senjuin (千手院) is held to be the earliest in origin, its name drawn from the Senjudo, a hall enshrining Senju Kannon (the Thousand-Armed Avalokitesvara) said to have stood at the western foothills of Mt. Wakakusa in Nara. The smiths who gathered in that quarter, working under the patronage and within the orbit of the great temples, formed the group, and their close association with the warrior-monks of the monastic establishments is thought to account for the scarcity of signed work that marks the school across its whole span. Old transmitted writings name two masters of the late period, Yukinobu and Shigehiro, though no secure example by either has been confirmed; the oldest surviving signed piece is a cut with the three-character inscription "Senjuin," placed in the early period. Blades prefixed Ko-Senjuin are assigned to the earliest phase and retain the lingering presence of the straight-sword (chokuto) tradition, material indispensable to the study of how the curved blade took form. Production runs from the late period through the era.
The school's vocabulary is rooted firmly in Yamato practice. The is an mixed with and passages of , often tending toward , with thick adhering well, abundant , and in finer examples a faint that leaves the steel bright and clear; the gathers and strengthens toward the edge on the most Yamato of the blades. The is fundamentally a with a shallow tendency, mixing , , and , the fraying into with uchi-noke, , , and worked in profusion; thick lies along a clear , while and run vigorously through the and . The enters straight and forms a with vigorous , or finishes in . Within this shared foundation the hands diverge. Yoshihiro is singled out as the smith whose and vary most, his especially strong, swelling from the school's quiet into a flamboyant large with and that reads toward . Sukemitsu gathers , , and small on a bright , a -leaning admixture set against the school's plainer temper, his bold two-character itself a mark of the hand. Nobuyoshi of the Ryumon group, a Yamato branch, divides between a lively carrying and a character and a subdued fraying into in which the Yamato color stands strongest.
To a Senjuin blade is to read its position among its neighbours, and the long-standing rule holds that within the Yamato Five Schools a of unusually vigorous and changeable character should be appraised as Senjuin, against the more restrained tempers of , Shikkake, , and Hosho. The in the , the , , and , and the or keep the work within Yamato, while the brightness of the and the on the edge set it apart from the plainer of its sister schools. Surviving forms span , , , , and , the notably numerous because they served as goshintai and ritual implements rather than weapons of the warrior class; many blades keep their , with pronounced and of archaic bearing. Yoshihiro's dated signatures give him a face among many anonymous hands, his manner the touchstone for -tinged attributions; Nobuyoshi is carried to high standing by a single great signed held as an imperial heritage; Sukemitsu is anchored by a dated reference in the Tokyo National Museum. The bulk of the school's record is , the attributions resting on construction, the flowing -inclined forging, and the bright alone, and provenance runs through religious institutions such as Tanzan Shrine and Kyoo Gokoku- (To-), attesting to the school's monastic roots.