Mon (門, gate) is a generic Japanese term for gate often used, either alone or as a suffix, in referring to the many gates used by Buddhist temples, Shinto shrines and traditional-style buildings and castles.
Unlike gates of secular buildings, most temple and shrine gates are purely symbolic elements of liminality, as they cannot be completely closed and just mark the transition between the mundane and the sacred. In many cases, for example that of the sanmon, a temple gate has purifying, cleansing properties.
Gate size is measured in ken, where a ken is the interval between two pillars of a traditional-style building. A temple's rōmon for example can have dimensions from a maximum of 5x2 ken to a more common 3x2 ken, down to even one ken. The word is usually translated in English as "bay" and is better understood as an indication of proportions than as a unit of measurement.
Like the temples they belong to, gates can be in the wayō, daibutsuyō, zen'yō or setchūyō style. They can be named after:
Not all such terms are mutually exclusive and the same gate may be called with different names according to the situation. For example, a Niōmon can also be correctly called a nijūmon if it has two stories.
Very different structurally from the others is the toriimon (normally called simply torii), a two-legged gate in stone or wood regularly associated with Shinto, but common also within Japanese Buddhist temples. As prominent a temple as Osaka's Shitennō-ji, founded in 593 by Shōtoku Taishi and the oldest state-built Buddhist temple in the country, has a torii straddling one of its entrances. The origins of the torii are unknown; although several theories on the subject exist, none has gained universal acceptance. Because the use of symbolic gates is widespread in Asia—such structures can be found for example in India, China, Vietnam, Thailand, Korea, and within Nicobarese and Shompen villages—historians believe it may be an imported tradition. It most often symbolically marks the entrance of a Shinto shrine. For this reason, it is never closed.