Term applied to medieval ecclesiastical architecture and referring to the deliberate use of differing pier forms in an arcade. Alternation is found in aisled churches throughout western Europe from the 11th to the 14th century. Its purpose is to articulate internal elevations through the subdivision of the main arcades and in some instances to emphasize certain liturgically important areas. In its simplest form the alternating system consists of the use of both the column (cylindrical) and the pier (square or rectangular in section). In antiquity these two types of support had specific functions that were almost always observed: the column supported the horizontal entablature and the pier supported the arch. By the Middle Ages this rule had been abandoned, and both types of support were used for arcades.
The earliest examples of alternation occur in the eastern Roman Empire during the 5th and 6th centuries ad. In most instances its use may be attributed to structural function, as at the church of ...