Around Keichō, the smith of the Echizen Shimosaka line was summoned to forge for Tokugawa Ieyasu and Hidetada, and as a result Ieyasu bestowed on him the character 康, whereupon he changed his name to Yasutsugu and was granted the right to cut the triple-hollyhock aoi crest on his tang. The published sources record that the line originated in Ōmi, moved to Echizen under the patronage of Yūki Hideyasu, and thereafter served the shogunal house in alternating years between Echizen and Edo, signing the long inscriptions that name the imported nanban-tetsu and Edo of Musashi. The first generation, who re-tempered fire-damaged Sōshū masterworks such as the Shishi Sadamune after the fall of Ōsaka, founded the house; this entry is read chiefly through the second generation, his legitimate heir Shimosaka Ichinojō, who took the tonsure as Kōetsu and died in Shōhō 3 (1646), and whose finest blades the published commentary holds difficult to tell from his father's.
The hand is read in the jigane first. It is an itame mixed with mokume that stands somewhat, in places flowing into nagare, with thick ji-nie, chikei entering, and at times jifu, the steel taking on the dark, blackish cast the published sources name as so-called Echizen steel. This dark, standing jigane is the constant across his work, and the commentary returns to it as the school's signature, noting again and again that the forging "displays the characteristics of Echizen steel"[[c:1]]. Over it the temper is built on notare into which gunome and ko-gunome are mixed, ko-ashi and yō entering, the nie strong with patches of coarse ara-nie gathering unevenly and scattering into the broken (basake) appearance that is his most personal effect; kinsuji and sunagashi run long, forming a striped course, the nioiguchi tending subdued (shizumi), with muneyaki along the back.
The bōshi is the surest single tell of the hand. It is most often a ko-maru, strongly swept in hakikake and turning back long, the published sources reading on one Jūyō katana that the temper "enters in midare-komi, sweeps strongly in hakikake, and turns back long with a ko-maru at the tip"[[c:2]]; on the quieter suguha pieces it shows the so-called Sanpin flavor with the same long, strong return. The rounded, swept ko-maru with its long kaeri recurs across the corpus, the point at times pointed (togari) or running in as midare-komi, but the long swept return is constant, and the judges read it together with the dark Echizen steel as the smith's own. A second native register sits beside the notare manner, and the published sources call it his intrinsic style: a suguha base, at times a medium suguha, into which small connected gunome are mixed, ko-ashi entering vigorously, the back generally hardened and the nioiguchi again subdued. On one such wakizashi the commentary names this the very type of his proper hand, "the intrinsic workmanship of the second-generation Yasutsugu"[[c:3]], the linked ko-gunome over a straight foundation, so that even where the temper quiets to suguha the steel keeps the smith's name.
The register the line is most celebrated for is its Sōshū-utsushi, the faithful copying of Sōshū masterworks at which the published sources say both the first and second generations excelled. The corpus holds reduced copies of the great meibutsu: an Ataka Sadamune katana carrying the gold-inlaid mark of the copy and a wakizashi reducing the same blade, and the only known second-generation copy of the Shishi Sadamune, whose burned original the first generation had re-tempered after the fall of Ōsaka. On these the forging tightens, the temper widens into notare mixed with gunome and large gunome and breaks into a large-pattern midare with tobiyaki and yubashiri toward a hitatsura feeling, the nie well adhered and the bōshi running almost as one sweep. On a tantō the judges read the deep nioi, the well-adhered nie and the kinsuji and sunagashi as an intent "aimed at the superior old Sōshū masters"[[c:4]], while observing that the standing jihada and the subdued nioiguchi still betray the Echizen hand. Distinct again is the carving: the deep, forceful Kinai-bori executed with the engravers Kiuchi Chisō and Kinai Tomochika, the three-deity figures, bonji, the Kurikara dragon, gomabashi and suken, by which the commentary even separates the generations, since the first generation's three deities are limited to Fudō Myōō with his two attendants while the second combines Jizō, Bishamonten and Monju.
What sets the second generation apart, in the judges' own framing, is how close he comes to the first while never quite reaching his scale. The commentary repeatedly measures him against his father: one katana it calls the work among his that "shows the style closest to the first generation"[[c:5]], and a late katana it praises as a blade in which "a level of skill not inferior to the first generation is demonstrated to the fullest"[[c:6]]. The signature itself carries the distinction, the form of the character 継 differing from the first generation's, and the line is read not against the old Sōshū it copied but by its own marks, the dark Echizen steel, the basake habuchi, the striped sunagashi, the long swept ko-maru return, and the deep Kinai carving.
Fujishiro grades him Jō-jō saku, and the Tōkō Taikan rates him at the upper end of the shintō field. He carries no designations at the highest national tiers; his record runs instead through twenty-four blades at the Jūyō rank and two katana designated Jūyō Bijutsuhin before the war, the latter recorded in the Yasutsugu Taikan and held in the Kurokawa Institute of Ancient Cultures. The provenance roll is daimyō and warrior: the Honda house, with ownership inscriptions of Honda Hida-no-kami and Honda Shichizaemon, several blades carrying gold-inlaid cutting-test inscriptions, one bearing the eccentric added line "Fuji hikashi," the smith's boast that even Mount Fuji stands low beside his sword. These are not blades that come to market often; with none held back at the highest protected tier yet most kept in long-standing collections, a signed Yasutsugu with the aoi crest appears from time to time and with patience, the broad, dark, powerfully built work the commentary calls full of "force and rustic flavor"[[c:7]] and well showing "the characteristics of Echizen work"[[c:8]].
