A clas (Welsh  "clasau") was a native Christian church in early medieval Wales. Unlike later Norman monasteries, which were made up of a main religious building supported by several smaller buildings, such as cloisters and kitchens, a clas was normally a single building. The building was run by a community of clergy and headed by an abod. Clasau were autonomous and were administered locally.
Following the Norman invasion of Wales in the late 11th century, many of the clasau of south Wales became dependencies of religious houses in England. This resulted in several sites becoming part of the Benedictine or Augustinian orders, or built upon in the following centuries by Norman churches.
Clas locations in Wales.
A map of "clasau" that can be recognised from Welsh documentary sources was provided by William Rees in 1951. Wendy Davies, in her study of the Llandaff Charters, has identified 36 monasteries or "clasau" from the 7th to 9th centuries, mainly in the Diocese of Llandaff, and a further 38 "ecclesiae" or churches, some of which would be monastic. Davies suggests that this is likely to represent about 50 "clasau" in total. This would suggest that if all the Welsh dioceses were similar and we include "clasau" which lie outside the modern boundary of Wales in the Welsh Marches, a total of 150-200 "clasau" may have existed. Many of these early "clasau" were granted as endowments to the monasteries, particularly those of the Cistercians, founded by the Normans and the Welsh Princes.
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