The Sa school of Chikuzen Province represents one of the most consequential departures in the history of Japanese swordsmithing. Its founder, Samonji -- commonly known as O-Sa or Dai-Sa -- was active in the early Nanbokucho period and is traditionally transmitted as the grandson of Sairen and the son of Jitsua. Since the Muromachi period, sword treatises have recorded him among the outstanding disciples of Masamune of Sagami, and though the question remains open, Soshu-den influence is unmistakably present in his work. Jitsua, the generation immediately preceding Samonji, had worked firmly within the classical Kyushu tradition -- a conservative idiom in which both ji and ha tended toward a subdued, archaic character with tempering built principally around suguha. Samonji broke decisively from this inherited manner, establishing a refined, sophisticated style "not previously seen among Kyushu-made blades" in which both jihada and hamon achieved a luminous brightness and clarity. From his forge emerged an extraordinary generation of disciples -- Yasuyoshi (traditionally identified as his son), Yukihiro, Yoshisada, Kunihiro, Hiroyuki, Hiroyasu, Sadayoshi, and Sadayuki -- each inheriting the master's approach and flourishing greatly during the Nanbokucho period.

The defining hallmark of the Sa school, recurring with near-universal consistency across the NBTHK's assessments of its members, is that "both ji and ha are bright and clear" (saeru). The characteristic forging displays ko-itame-hada mixed with mokume, often with areas of nagare-hada and a tendency toward hada-dachi (standing grain). Extremely fine ji-nie adheres thickly, abundant chikei enliven the ground, and nie-utsuri appears in many examples. The hamon is characteristically founded on notare mixed with gunome, ko-gunome, and pointed togari-ba elements; the nioiguchi is deep, nie attaches conspicuously, and kinsuji and sunagashi run vigorously throughout, accompanied by yubashiri and tobiyaki. The boshi is among the school's most diagnostic features: it rises in tsukiage, the tip becomes pointed with vigorous hakikake, and the turnback is characteristically long -- a hallmark that "conveys great force." Within this shared vocabulary, individual members developed recognizable variations. Yukihiro's work approaches Samonji's most closely, to the point that the NBTHK acknowledges certain works may represent daisaku (substitute workmanship) by Yukihiro bearing Samonji's name. Yasuyoshi introduced a distinctive Bizen coloration into the Sa idiom -- a nioi-dominant manner with ko-nie and bo-utsuri that can "at a glance be mistaken for Osafune works such as those of Kanemitsu." Kunihiro is distinguished by the boldest and most vigorous midare, with the yakiba taken at its highest level, while Yoshisada favored a comparatively calm and restrained character with small-patterned designs. Hiroyuki's tempering tends toward a relatively quiet irregularity, and Hiroyasu's work is most often identified by the conspicuous prominence of gunome within the school's characteristic temper. Most extant signed works across the school are tanto and small wakizashi -- signed tachi are exceedingly scarce, with only the celebrated National Treasure meibutsu Kosetsu Samonji widely acknowledged among Samonji's own hand.

The Sa school's historical significance lies in its role as the vehicle through which the Soshu-den aesthetic was transplanted to Kyushu, transforming the region's swordmaking tradition in a single generation. Samonji himself is consistently characterized by the NBTHK as having "cast off the earlier classical Kyushu sphere," and his finest works are termed kessaku that leave "none of Samonji's highlights unshown." Yasuyoshi's relocation from Chikuzen to Choshu (Nagato Province) -- confirmed by a tanto dated Shohei 17 inscribed "Choshu-ju Yasuyoshi" -- extended the school's geographic reach and established a branch that continued into the Muromachi period. Many Sa blades were transmitted within the most distinguished daimyo collections of Japan -- the Owari Tokugawa, the Sendai Date, the Tachibana of Yanagawa, the Bizen Ikeda, and the Mori families -- with several recorded in the Kyoho Meibutsu-cho. Within the Sa group, the school's vigor, clarity, and technical command constitute an aesthetic achievement that the NBTHK evaluates at the highest level of the Japanese sword tradition.