A tanzen obi is a small, lightweight belt used to tie together a tanzen (thick, padded jacket), haori (kimono jacket), and a kimono.
These are also oftentimes used in places like an onsen (public bath) or ryokan (Japanese-style inn), typically with a yukata. These should not be worn formally and should only be used indoors. A heko obi or, better yet, a kaku obi should be worn by men when outdoors.
Due to their size and oftentimes being fairly malleable, tanzen obis are sometimes used as a datejime (a strap underneath an obi to hold up the kimono).

This specific tanzen obi appears to be from the Zaimoku Sakaeya Ryokan (材木栄屋旅館) in Yamagata Prefecture, to the west of Sendai.

Width: 8cm (3")
Length: 239cm (94")

Age:
Before 2002

Quality:
Silk & wool, thick, stiff
B - General wear & tear

Cleaning:
Dry Clean Only!