Japanese traditional formal attire generally displays the mon of the wearer. Commoners without mon often used the mon of their patron or the organization they belonged to. In cases when none of those were available, they sometimes used one of the few mon which were seen as "vulgar", or invented or adapted whatever mon they wished, passing it on to their descendants. It was not uncommon for shops, and therefore shopowners, to develop mon to identify themselves.
Rules regulating the choice and use of mon were somewhat limited, though the selection of mon was generally determined by social customs. It was considered improper to use a mon that was known to be held by someone else, and offensive to use a mon that was held by someone of a high rank. When mon came into conflict, the lower-ranked person sometimes changed their mon to avoid offending their superior. The mon held by the ruling clans of Japan, such as Tokugawa's hollyhock mon and the Emperor's chrysanthemum mon, were legally protected from unauthorized usage.
There are no set rules in the design of a mon. It most commonly consists of a roundel encircling a figure of plant, animal, man-made, natural or celestial objects, all abstracted to various degrees. Religious symbols, geometric shapes and kanji were commonly used as well.
Similar to the blazon in European heraldry, mon are also named by the content of the design, even though there is no set rule for such names. Unlike in European heraldry, however, this "blazon" is not prescriptive - the depiction of a mon does not follow the name - instead the names only serve to describe the mon. The pictorial depictions of the mon are not formalized and small variations of what is supposed to be the same mon can sometimes be seen, but the designs are for the most part standardized through time and tradition.
The degree of variation tolerated differ from mon to mon as well. For example, the paulownia crest with 5-7-5 leaves is reserved for the prime minister, whereas paulownia with fewer leaves could be used by anyone. The imperial chrysanthemum also specifies 16 petals, whereas chrysanthemum with fewer petals are used by other lesser imperial family members.
Japanese heraldry does not have a cadency or quartering system, but it is not uncommon for cadet branches of a family to choose a slightly different mon from the senior branch. The princely families (Shinnouke), for example, each uses a modified chrysanthemum crest as their mon. Mon holders may also combine their mon with that of their patron, benefactor or spouse, sometimes creating increasingly complicated designs.
Mon are essentially monotone; the colour does not constitute part of the design and they may be drawn in any colour.