Obi are the top-most sash worn with various styles of Japanese clothing; several other sashes may be worn under the obi to secure the clothing in place and/or to enhance the shape of the body or of the uppermost sash.
Except in martial arts, obi are always tied at the back — traditionally one knotted the obi in its final position, but many people now tie the obi at the front where the knots can easily be seen, and then twist the sash into the correct position. In former times, courtesans and prostitutes wore obi tied at the front for easy removal and retying.
Kimono are full-length robes traditionally worn by both men and women in Japan. Though there are many different types of kimono, obi are worn with all of them, and function both to keep the robe closed and in place on the body, and as decoration. Obi were traditionally made of silk, and silk is still the most traditional and formal material for obi.
Men’s kimono obi are rectangular, usually about five or six centimetres wide and two metres or more in length. They are usually seen in dark colours such as black and blue, and typically have little decoration, though decorative stitching in a contrasting colour, often white, is common. Men’s obi are wrapped one or more times around the waist and are tied slightly off-centre at the back, most commonly in the style of knot called kai no kuchi (貝の口), literally “clam’s mouth,” though there are other knots as well. Men’s obi are normally worn quite low on the waist, somewhat below the stomach.
Women’s kimono obi are twice as wide as men’s or more, and are frequently very decorative and colourful, though this will depend on the age of the wearer, the type and style of kimono, the season, and the occasion for which it is being worn.
Women’s obi are also wrapped one or more times around the waist, and are not only much wider but are tied much higher up, generally with the top just under the breasts. Women also use various styles of knots, including the kai no kuchi, but more typically women’s obi are tied in a so-called drum knot, a large roll at the back, usually enhanced and shaped with padding called makura, literally “pillows.” Unlike men, women tie the knot at the centre of the back.