The obi remains one of the most elaborate parts of traditional Japanese fashion. It began as a simple sash a few inches wide that served purely to tie clothes in place. However, much like the kimono itself, it evolved into a fashion statement during the Edo period, and what was once a plain strip of cloth became a foot-wide band of brocade fabric requiring additional cords to tie it in place. Obis were originally tied in the front, but with knots rapidly growing in complexity, they shifted to the back to allow more space for elaborate displays and greater comfort. Wide obis also allowed the hiding of the "ohashori," or the fold created by tucking the kimono up into its belt to raise the hem. (Additionally, tying the obi in the front became a symbol of prostitution, since it was easier for someone to repeatedly tie and untie their clothes if the obi knot was in front.)
Women's obis are the most obvious and flagrant, and they always stand out from the kimono. An obi is never chosen with the same fabric as the kimono, because part of the purpose is to serve not just as a belt, but also as an accessory in the same way a necklace or a hairpiece would. They are chosen to complement the colors of the robe and stand out on their own. These wide belts are long enough to wrap around the waist with a generous portion of length hanging behind, and the dangling ends are tied behind the back in an elaborate knot depending on the fashion preferences of the wearer.
Obi knots are as much of an art as the obi itself, and some of them even require pillows and stiffeners to shape them and help them keep their structure through daily wear. The knowledge of obi knots has faded since its heyday in the 18th and 19th centuries, and for the most part only historians and "kimono-wearing schools" know how to recreate the most elaborate antique shapes. In the modern day, most wear their obi in a simple drum knot or use a "tsuke obi," which has a pre-tied knot. Tsuke obis have a short cord that ties the belt together behind the back, and then a fake knot is clipped over this cord with a wire hook.
Men's obis are simpler than women's and are only four inches wide. Tied in a simple knot at the back of the waist, it serves a much less decorative purpose than a women's obi. Also, while there are over ten types of obi for women, men have two: the heko and kaku obis. Heko obis are soft, informal, and worn by boys and young men. Older men wear the kaku obi, a simple silk belt suited for any formality and tied in a flat knot called a "kai no kuchi."
Children tend to wear smaller versions of the obis that adults wear, unless they are so small that they need gentler cloth instead of a proper obi. In that case, their belt resembles a sash or scarf and is made of soft cotton and tied in a simple square knot. Tsuke obi are also common in children's outfits because of how easy they are to tie and remove.