An obi, which means "sash", is a belt tied around the waist of a woman's kimono to keep it in place. About 400 years ago, however, the obi was just a thin cord used as a waist tie.
The width of the obi became slightly wider at the beginning of the Edo period (around 1600) and developed into its present form in the middle of the Edo period (around 1700).
In the early days, an obi was a cord or ribbon-like sash about 8 cm wide. Men's and women's obi were similar. By the early 17th century, both women and men wore a ribbon obi. By the 1680s, the width of the obi for women had doubled from its original size.
By the 1730s, women's obi were about 25 centimetres wide, and by the turn of the 19th century they were as wide as 30 centimetres. At that time, separate ribbons and cords were already necessary to hold the obi in place. The men's obi was widest in the 1730s at about 16 centimetres.
Towards the end of the 18th century, it was considered very fashionable to wear an extremely long kimono or kosode, which was dragged behind the wearer in the house.
To prevent this long hem from dragging on the ground outdoors, the excess fabric was gathered at the waist and held in place with a shigoki obi underneath.
Nowadays, kimonos are still made excessively long because they are essentially one size fits all. It's easy to tuck away excess fabric, but much harder to add fabric, so kimono makers prefer to play it safe and make them excessively long to fit everyone.
The excess fabric is gathered at the hips, creating a pleat called the ohashori, which is then hidden behind the obi, although nowadays part of the pleat can be seen under the obi.