The Yoshii school (吉井) took its name from a locality on the Yoshii River in province, the stretch of country facing across the water, and it is from that base that the lineage is read in the late period, through the , and on into the . Tradition holds that the line began with Tamenori toward the close of the age, though pieces reaching that far back are exceedingly rare; the separates the older tier, of date and before, as , reserving the plain name Yoshii for the full production. The dated, signed hands by whom the school is most clearly told run from Kagenori and Noritsuna in the fourteenth century, through Morinori and Naganori of the late into Oei, to Kiyonori in the middle , the name passing along a chain of Nori- and Tsugu-cut signatures (Yoshinori, Kiyonori, Naganori his son) that fixes the school to the year across roughly a century and a half. Kagenori, whose lineage-name endured longest and is held by one account to be the main line, anchors the early record; Kiyonori supplies its textbook face.
What binds the school as a body, and what no other branch of - shares in the way, is a single distinctive temper: a small (the or the trade calls the "Yoshii-ba") linked in a regular, orderly, repeating run, often with a slight slant or and pointed elements folded into the row. Over this the school raises its second great hallmark, a so particular that the published sources describe the very shape of the hardened edge as though cast straight into the as a shadow, echoing the outline of the temper rather than drifting as an ordinary reflection would. The is a tight, standing mixed with , at times taking an -like or tendency that itself reads as a Yoshii tell, with densely applied and entering; the follows the temper into the point in , turning in . Within this shared vocabulary the generations diverge in degree: the older hands work -laden, with vigorous and that on Kagenori's boldest recall the oldest , while the production is mostly -based with modest and a soft . A quieter second register runs throughout the school, a narrow carrying only a slight admixture of small , the calm half of the hand; even there it is the folded-in that returns the attribution to Yoshii. The long signatures, cut toward the , frequently use -, reverse-driven chisel strokes that mark the very as the school's own.
To a Yoshii blade is therefore to read the regular linking first: the orderly, repeating and the shadow-cast distinguish it at once from the softer, cloud-like and the broader of the mainstream, and the strength of the on older pieces is what guards against a later confusion with . The school's standing rests less on grand provenance than on this recognizability and on its documentary value, the dated inscriptions making its smiths placeable to the year, which for late hands is much of the appeal. Among the named members Kagenori and Morinori carry the line at its finest, the Fujishiro appraisers rating Morinori and Naganori at chu-jo , a step above the school's average run; Kiyonori's best is judged to approach the range within a frame. Recorded whereabouts scatter across temple, museum, and private hands, a Kagenori in the Sano Art Museum, a Kiyonori at Yasukuni Shrine, a Naganori in the Tokugawa Art Museum, and a Kagenori once appraised as passing through the Date family; none rises above the Important Sword tier into National Treasure or Important Cultural Property. A signed, dated Yoshii blade thus remains an attainable way to hold late - in its most exactly knowable and most immediately recognizable form.