An obi (帯) is a belt of varying size and shape worn with both traditional Japanese clothing and uniforms for Japanese martial arts styles. Originating as a simple thin belt in Heian period Japan, the obi developed over time into a belt with a number of different varieties, with a number of different sizes and proportions, lengths, and methods of tying. The obi, which once did not differ significantly in appearance between men and women, also developed into a greater variety of styles for women than for men.