In Chikuzen Province, on the northern coast of Kyushu, the Sa school (左, the Samonji line) took shape in the early Nanbokucho period as a deliberate break with the Kyushu work that came before it. Its founder, the smith who cut the single character 左 on his tang, is read in the published sources as Saemon Saburo, commonly called O-Sa or Samonji; he is placed as the grandson of Sairen and son of Jitsua, the old Chikuzen line whose steel, in the words the NBTHK restates blade after blade, ran sunken and rustic in a restrained suguha inherited from Yamato. The Muromachi sword books count O-Sa among the pupils of Masamune, and his study under the Soshu-den masters of Sagami is the pivot of the school's identity: against his forebears' provincial manner he is the reformer who established a clear, unprovincial midare, lifting Chikuzen work to a rank it had not before held. The single-character 左 signature, often paired with 筑州住 on the reverse, became the mark of a lineage rather than one hand, and around the founder a large body of pupils formed.

What the Sa smiths held in common is a steel that runs bright and clear where the older Kyushu work ran dark. The shared vocabulary is a well-knit itame, tight ko-itame on the small tanto and standing more open on the grand katana, carrying dust-fine ji-nie and frequent dark chikei, the recurring institutional verdict being that ji and ha alike run limpid and lucid, free of the whiteness or blackness of lesser Kyushu steel. Over that ground the school tempers a Soshu-den notare mixed with gunome and ko-gunome, the nie deep and the nioiguchi bright, with ashi and yo entering and fine kinsuji and sunagashi drawn through the ha; the boshi thrusts up (tsukiage) to a pointed tip with a long brushed return, the one structural trait the judges look to across the whole line. Within that idiom the hands diverge by degree. O-Sa stands alone for the brilliance of his bright Chikuzen ji-nie and forceful pointed boshi; among the pupils and the later Sue-Sa generations the published sources read individual registers, Yukihiro set nearest the master in refined ko-itame, Yasuyoshi carrying a Bizen temperament with a whitish utsuri that strays toward Kanemitsu, Kunihiro the boldest and highest-tempered midare, Yoshisada the smallest and most compact pattern, Sadayoshi and Hiroyasu the quieter suguha-toned manner, and Hiroyuki a continuous restrained gunome. The school as a whole carries the Sagami manner into Kyushu and transforms it.

A collector seeks Sa-mon work for what the founder accomplished and for how legibly the school can be read. Kantei proceeds first through the ji and the point: the bright, clear ji-nie free of whiteness is what tells a Sa blade from its Soshu fellows when the thrusting, pointed boshi alone will not, and the whole record divides cleanly into signed hira-zukuri tanto of fine flowing chisel and osuriage-mumei katana papered as Den Sa or simply to the school. O-Sa's own standing is the highest the Kyushu tradition reached, a Sai-jo saku hand whose register runs from the small signed tanto to the wide Nanbokucho katana held together as one hand by that limpid ji and ha; signature works such as the Kosetsu Samonji, his only signed tachi to survive, and named meibutsu held in the Owari Tokugawa, Date, Sakai, Tachibana, and Uesugi houses anchor his place, one katana having passed through Hideyoshi's hands. The pupils support that standing as members of the line rather than rivals to it: a signed Yasuyoshi, Yukihiro, or Yoshisada, or a securely papered Den Sa katana attributed to Kunihiro, Hiroyasu, Hiroyuki, or Sadayoshi, comes to open hands only rarely, and a signed O-Sa tanto is a landmark of a collection when it appears. For the student of Soshu-den, the Sa school is the place where the Sagami manner becomes a Kyushu one, and a Sa-mon blade in private keeping is a tangible record of that transformation.