What is Tanagoharagata in Japanese Katana terminology?

The term "Tanagoharagata" (たなご腹形, たなごはらがた) refers to a specific shape of the tang (Nakago) of a Japanese sword. This design is characterized by a significantly narrowed lower end of the tang, with the central part of the tang bulging out. This shape is named "Tanagoharagata" because it resembles the belly of a bitterling fish (Tanago in Japanese). This shape of the tang is representative of the works of the swordsmith Muramasa and his school. However, similar shapes can also be found in the works of swordsmiths from Shimada, Shimohara, and Sagami provinces. This terminology is used to describe the specific characteristics of the sword's structure, contributing to its overall aesthetic and functional design.