Intercalary months are a feature of the Chinese-type luni-solar calendars, which Japan used untill 1873. Because 12 lunar months are shorter than one solar year, every few years an extra month, called an intercalary, or leap, month (閏月 uruu-tsuki or jungetsu) is inserted to keep the lunar calendar related to the solar (seasonal) calendar. In this wiki the intercalary months are indicated by "int." The intercalary month can be inserted after any month of the year, and that year thus has 13 months, though there is no "13th month". For example, in 1599 an intercalary month was inserted after the 3rd month, so that year the months were: 1599/1, 1599/2, 1599/3, 1599/int.3, 1599/4, 1599/5, etc. to 1599/12. For more information see on the Japanese calendar.