oyabun-kobun where kobun (lit. foster child) owe their allegiance to the oyabun (lit. foster parent). In a much later period, the code of jingi (justice and duty) was developed where loyalty and respect are a way of life.
The oyabun-kobun relationship is formalized by ceremonial sharing of sake from a single cup. This ritual is not exclusive to the yakuza—it is also commonly performed in traditional Japanese Shinto weddings, and may have been a part of sworn brotherhood relationships.
During the World War II period in Japan, the more traditional tekiya/bakuto form of organization declined as the entire population was mobilised to participate in the war effort and society came under strict military government. However, after the war, the yakuza adapted again.
Prospective yakuza come from all walks of life. The most romantic tales tell how yakuza accept sons who have been abandoned or exiled by their parents. Many yakuza start out in junior high school or high school as common street thugs or members of bōsōzoku gangs. Perhaps because of its lower socio-economic status, numerous yakuza members come from Burakumin and ethnic Korean backgrounds.
Yakuza groups are headed by an Oyabun or Kumichō (family head) who gives orders to his subordinates, the kobun. In this respect, the organization is a variation of the traditional Japanese senpai-kōhai (senior-junior) model. Members of yakuza gangs cut their family ties and transfer their loyalty to the gang boss. They refer to each other as family members - fathers and elder and younger brothers. The Yakuza is populated almost entirely by men, and there are very few women involved who are called "nee-san" (older sister). When the Yamaguchi-gumi (Family) boss (Kazuo Taoka) was shot in the early 1980s, his wife (Fumiko) took over as boss of Yamaguchi-gumi, albeit for a short time.
The Yakuza have a very complex organizational structure. There is an overall boss of the syndicate, the kumicho, and directly beneath him are the saiko komon (senior advisor) and so-honbucho (headquarters chief). The second in the chain of command is the wakagashira, who governs several gangs in a region with the help of a fuku-honbucho who is himself responsible for several gangs. The regional gangs themselves are governed by their local boss, the shateigashira.
Each member's connection is ranked by the hierarchy of sakazuki (sake sharing). Kumicho are at the top, and control various saikō-komon (senior advisors). The saikō-komon control their own turfs in different areas or cities. They have their own underlings, including other underbosses, advisors, accountants and enforcers.
Those who have received sake from oyabun are part of the immediate family and ranked in terms of elder or younger brothers. However, each kobun, in turn, can offer sakazuki as oyabun to his underling to form an affiliated organisation, which might in turn form lower ranked organisations. In the Yamaguchi-gumi, which controls some 2,500 businesses and 500 yakuza groups, there are even 5th rank subsidiary organisations.