Yasutsugu designates a lineage of -era swordsmiths based in Fukui, Province, who served as official swordsmiths to the Tokugawa shogunal house. The founder received the character "" from Tokugawa Ieyasu and was further permitted to engrave the (hollyhock crest) on his works, earning the epithet "-shita ." His legitimate heir, the second generation, thoroughly inherited the style of the first generation. Following the second generation's death in Shoho 3 (1646), a succession dispute arose between his heir Umasuke and his uncle Shiro'emon. Ultimately, Umasuke inherited as the third head serving the bakufu, while Shiro'emon established a separate house as " third-generation Yasutsugu," entering the service of the Matsudaira family. In this way the Shimosaka Yasutsugu line divided into two hereditary houses.
The forging characteristically shows tending toward a standing grain with a dark tone; adheres well and enter. The typically favors or a notare-based pattern with mixed in; the tends to be deep and somewhat subdued, with adhering strongly and forming clusters; often appears abundantly. The is generally or shallowly with a turn-back. Many works are embellished with forceful , including formal , , , and descending dragons. The third generation was relatively prolific, producing predominantly , with being few and pieces in or remaining rare.
The Yasutsugu line is distinguished by its close association with the ruling Tokugawa authority, and works frequently bear cutting-test inscriptions by noted examiners such as Yamano Ka'emon Nagahisa. The first generation was particularly celebrated for his skill in producing copies of famous blades, and the line as a whole consistently displayed the characteristic features of work across successive generations.