obi is not really a belt
. It would be a crime to call it one. Sash
is probably a better word.
Obi are tied around kimono but do not actually hold the kimono together. All kimono have small cords which are tied together. An obi is mostly for decoration, hiding the material at the waist which becomes creased and a little bunched. It can be tied tightly to reduce the size of the waist for women.
Like kimono, obi are of varying quality. It is important that the obi are of at least the same quality as the kimono - great care has to be taken to ensure that the obi is not of inferior quality, or clashes with the main ensemble. Geisha spend much time choosing which obi to wear with their kimono. Naturally they have the finest available, hand-woven from silk and designed with intricate patterns.
Obi are normally tied at the back. Various types of knots are used to secure it. Geisha and maiko have their knots tied differently to indicate their rank. Obi have been known to be tied at the front. However this was only ever the case for prostitutes - the tying of the knot at the front indicated "easy access".