This study examines a mysterious item of the Ainu women’s undergarment—the upsor kut, or chakh chanki, which, in ethnographic collections and scholarly texts, is described as a “belt of modesty”. A comparative and historical analysis of Ainu women’s girdles from Hokkaido and Sakhalin was carried out. They are displayed in very small numbers at museums of Russia, Japan, and the UK. These artifacts are rare, as women had to preserve their upsor kut (chakhchanki) from being seen by strangers, especially males. They became a part of late 19th to early 20th century ethnographic collections, because scholars, such as B.O. Piłsudski and N.G. Munro, became trusted by the natives. In the past, Japan’s hard-line policy of assimilation for indigenous peoples, the banning of the Ainu language and traditional culture, and the introduction of schooling and public health service resulted in an even greater secrecy of Ainu women and the gradual decline of the tradition of wearing secret girdles, precluding the carrying out of fi eld studies. The analysis of Ainu linguistic and folkloric materials analyzed by Japanese and European researchers sheds light on the function and meaning of these items of the women’s undergarment. In essence, they had two important functions: determining the maternal lineage and protecting the family and the clan. This suggests that remnants of matrilineal exogamy existed in Ainu patriarchal society, which eventually disappeared at the turn of the 20th century.