Y?gen gaisha are based on the German GmbH and were implemented in Japan in the Limited Company Act () of 1940. The Companies Act of Japan, implemented on May 1, 2006, replaced the y?gen gaisha with a new form of company called g?d? gaisha, based upon the American limited liability company. Following the implementation, no new YKs were allowed in Japan, but pre-existing YKs were allowed to continue their operations as kabushiki gaisha under special rules.
Whether the term is pronounced as y?gen gaisha or y?gen kaisha is up to the local dialect or the company's preference when it is part of the company's name. While it is pronounced y?gen gaisha in standard Japanese, the alphabetic abbreviation is always Y.K. by standard.
As of 2005, a Y.K. can have up to 50 investors, called members (, shain). The members were required to provide at least ¥3 million in capital contributions, with each investment unit (, mochibun) valued at no less than ¥50,000. The minimum capital amount was much more permissive than the ¥10 million minimum for a kabushiki gaisha. A Y.K. was also not required to issue certificates for investment units, whereas stock certificates were required for a K.K.
Unlike a K.K., a Y.K. does not need to have a board of directors or statutory auditors: the minimum requirement is one director (, torishimariyaku).
Because of its simplified structure and relatively lax incorporation requirements, the Y.K. form is associated with small businesses. However, some larger companies have used the form: ExxonMobil's principal Japanese subsidiary, for instance, is a Y.K. with paid-in capital of ¥50 billion (US$420 million). In addition to simplified corporate governance, a Y.K. receives some tax benefits under foreign laws such as the U.S. Internal Revenue Code.