Literally, "hall" (dō 堂) for the "sangha" (sō 僧). Because sō 僧 can also be translated as "monk," sōdō 僧堂 is often rendered in English as "monks' hall." That is not an error, but "sangha hall" is a more apt translation, for two reasons. First, all of the buildings in a monastery are for use by monks, but the sōdō is the place where only the great assembly of monks (daishu 大衆) - a "sangha" in the sense of a "group" or "collective" - is quartered. Monks who hold various monastic offices are not part of the great assembly: they have their own quarters (ryō 寮) where they perform their duties, keep their personal possessions, and sleep at night. Secondly, the sōdō has traditionally been considered one of the three most important buildings in a monastery, the first two being the buddha hall (butsuden 佛殿) and the dharma hall (hattō 法堂). Because the "three treasures" (sanbō 三寶) are the "Buddha, dharma, and sangha" (buppōsō 佛法僧), the third building in this set is best called the "sangha hall."
1. In Song and Yuan dynasty Chinese monasteries and the medieval Japanese Zen monasteries (such as Dōgen's Eiheiji) that were modeled after them, the sangha hall was the central facility on the west side of a monastery compound. It was a large structure divided internally into an inner and an outer hall and surrounded by enclosed corridors that connected it with nearby ancillary facilities. The inner hall was further divided into front and rear sections and featured low, wide sitting platforms arranged in several blocks in the center of the floor space and along the walls. Enshrined on an altar in the center of the inner hall was an image of Monju Bodhisattva, called the Sacred Monk, who was treated both as the tutelary deity of the hall and the highest ranking "monk" in the assembly. Registered monks of the great assembly spent much of their time at their individual places on the platforms, sitting in meditation, taking their meals, and spreading out bedding for sleep at night. Their bowls were hung above their seats, and their few personal effects and monkish implements were stored in boxes at the rear of the platforms. Seats in the inner hall were also designated for the abbot and the monastic officers and assistants who directed the training there. Monks with no special duties were seated in order of seniority, according to years elapsed since ordination. Other officers, acolytes, and unregistered monks were assigned seating places in the outer hall, where the platforms were not deep enough to recline on. They would gather in the sangha hall for meals, ceremonies, and a few periods of meditation but slept elsewhere. Observances centered in the sangha hall included: recitations of buddha names to generate merit in support of prayers; rites marking the induction and retirement of monastic officers in the ranks of stewards and precepts; novice ordinations; sutra chanting; prayer services sponsored by lay patrons, who would enter the hall to make cash donations and hear their prayers recited; and formal tea services. Apart from those group observances, however, the individual drinking of tea, sutra reading or chanting (whether for study or devotional purposes), and writing were not allowed in the sangha hall, lest they interfere with the attitude of introspective concentration that monks were supposed to maintain there. Monks of the great assembly could engage in such activities only at their seats in the common quarters. Contrary to the claims of some modern scholarship, sangha halls were a standard feature of all major Buddhist monasteries in Song and Yuan dynasty China. The modes of practice that went on in them were neither invented by nor unique to monks belonging to the Zen school.
In Kamakura period (1185-1333) Japan, there were a few Chinese-style monasteries not associated with the Zen tradition that had sangha halls, but Zen monks such as Dōgen and Keizan were in the forefront of the movement to implement sangha hall training. During that period, most Zen monasteries in Japan had sangha halls, but the divisiveness of competing lineages and the proliferation of mortuary sub-temples (tatchū 塔頭) in the Muromachi period (1333-1573) resulted in the demise of those facilities for communal training. It was not until 1796 that a Song Chinese style sangha hall was rebuilt at Eiheiji and an effort was made to reinstate the modes of training there that had originally been established by Dōgen. Even today, there are only a handful of Song style sangha halls operating in Japan, all of them at Soto monasteries (Eiheiji and Sōjiji first among them).
2. In Edo period (1600-1868) Japan, the term sangha hall (sōdō 僧堂) came to refer to any Zen temple that operated as a training monastery, that is, a place with a meditation hall (zendō 禪堂) and a sizable community of monks in training under a Zen master, as opposed to an ordinary parish temple which typically housed just a resident priest and a few disciples. In the revival of communal monastic practice that was sparked by the importation of so-called Ōbaku Zen from China in the seventeenth century, ordinary temples were converted into training monasteries (sōdō 僧堂) by "opening platforms" (kaitan 開單), which is to say, building meditation halls.