紋, also called 紋章, 紋所, and 家紋, are Japanese emblems used to decorate and identify an individual, a family, or (more recently) an institution or business entity. While mon is an encompassing term that may refer to any such device, kamon and mondokoro refer specifically to emblems that are used to identify a family. An authoritative mon reference compiles Japan's 241 general categories of mon based on structural resemblance (a single mon may belong to multiple categories), with 5,116 distinct individual mon. However, it is well acknowledged that there are a number of lost or obscure mon. Among mon, the mon officially used by the family is called 定紋. Over time, new mon have been created, such as 替紋, which is unofficially created by an individual, and 女紋, which is created by a woman after marriage by modifying part of her original family's mon, so that by 2023 there will be a total of 20,000 to 25,000 mon.
The devices are similar to the badges and coats of arms in European heraldic tradition, which likewise are used to identify individuals and families. Mon are often referred to as crests in Western literature, the crest being a European heraldic device similar to the mon in function.
Mon originated in the mid-Heian period (900–1000) as a way to identify individuals and families among the nobility. They had a pecking order, and when bullock cart passed each other on the road, the one with the lower status had to give way, and the mon was painted on the gissha. The Heiji Monogatari Emaki, an picture scroll depicting the Heiji rebellion, shows mon painted on gissha. Gradually, the nobility began to use mon on their own costumes, and the samurai class that emerged in the late Heian period and came to power in the Kamakura period (1185–1333) also began to use mon. By the 12th century, sources give a clear indication that heraldry had been implemented as a distinguishing feature, especially for use in battle. It is seen on flags, tents, and equipment.