The term “cloister” has two definitions, both based on the Latin word claustrum (pl. claustra), meaning an enclosure, a lock, or a place that is shut. A cloister may be the monastery or convent that encloses a religious community away from the world, monks as well as canons, priests living together under a rule. Architecturally, a cloister is the open courtyard that connects the various buildings of the monastery by means of a covered walkway. Cloister galleries are usually formed by an arcade of columns springing from a plinth, often with piers at the corners. These may be ornamented with carvings, as on the column capitals and sometimes on the columns themselves. The cloister courtyard normally includes a well and garden. By the 9th century, the layout for an ideal monastery had been recorded in the Plan of St. Gall. There, the cloister is rendered in what was to become its traditional form, a quadrangle. The main features of the St. Gall plan are found, for example, at Monte Cassino, the head of the Benedictine order, and at the monastery of Cluny. This arrangement was generally followed by Benedictine and Cistercian monasteries. To the north of the cloister lies the church nave; to the east are the church transept and dormitory. Along the east gallery is the chapter house, the meeting place for the monastic community, then there is a parlor near a spring that must have served for washing, baths, and latrines. To the south is the refectory, and to the west, the gateway and guesthouse. The north gallery of the cloister often has a funerary function, with burials of abbots located there. The west gallery often flanks the cellar and guesthouse. Because the cloister provides the passageway between the church, dormitory, refectory, and chapter house, it is the heart of a monastery, and the decoration reflects its spiritual ideals. On the one hand, it was the site for processions on Sundays and holidays, for teaching novices and children, and for reading and meditation. On the other hand, the cloister may be used for a variety of activities, some concerning the practical side of life, such as laundry, barbering, and the reception of guests. Larger communities sometimes had a second cloister connecting the infirmary to the rest of the compound. Because monasteries and colleges of secular canons sometimes evolved into universities, campuses often have cloister garths as well. This article focuses on medieval cloisters.