Although it is still found today, mimetic ornament is by far the commonest type of architectural ornament in Asian civilizations and generally throughout antiquity. It grows out of what seems to be a universal human reaction to technological change: the tendency to reproduce in new materials and techniques shapes and qualities familiar from past usage, regardless of appropriateness. This tendency may be called the principle of mimesis. Most common building types in antiquity, both East and West (e.g., tombs, pyramids, temples, towers), began as imitations of primeval house and shrine forms. An obvious example is the dome, which developed as a permanent wooden or stone reproduction of a revered form originally built of pliable materials. In the mature stages of early civilizations, building types tended to evolve beyond early prototypes; their ornament, however, usually remained based on such models. Decorative motifs derived from earlier structural and symbolic forms are innumerable and universal. In Indian and Chinese architecture, domical and other originally structural forms occur often and lavishly as ornament. In ancient Egypt, architectural details continued to preserve faithfully the appearance of bundled papyrus shafts and similar early building forms. In ancient Mesopotamia, brick walls long imitated the effect of early mud-and-reed construction. In the carved-stone details of the Greco-Roman orders (e.g., capitals, entablatures, moldings), the precedent of archaic construction in wood was always clearly discernible.

The prevalence of mimetic ornament in architecture may be explained in two ways. Some is religious in origin. Certain forms and shapes, through long association with religious rites, became sacred and were preserved and reproduced for their symbolic value. These forms continued to be understood even though they were often stylized into abstract or geometric patterns, unrecognizably removed from their naturalistic models. Much mimetic ornament, however, even in early times, can be ascribed simply to inertia or conservatism. People have generally tended to resist change; they find it reassuring to be surrounded by known and familiar forms. Reproducing them as ornament on newly introduced forms is a common reaction to the vague feeling of uneasiness that rapid social and technological change induces; it provides a satisfying sense of continuity between the past and the present. This resistance was a factor in the 19th- and early 20th-century practice of disguising new techniques of construction in metal and glass by an overload of ornament imitating earlier styles.

Applied ornament

Architectural ornament in the 19th century exemplified the common tendency for mimetic ornament, in all times and places, to turn into mere applied decoration, lacking either symbolic meaning or reference to the structure on which it is placed. By the 5th century bce in Greece, the details of the orders had largely lost whatever conscious symbolic or structural significance they may have had; they became simply decorative elements extrinsic to the structure. The Doric frieze is a good case: its origin (i.e., an imitation of the effect of alternating beam ends and shuttered openings in archaic wood construction) remained evident, but it came to be treated as a decorative sheath without reference to the actual structural forms behind. In losing their mimetic character, the details of the Greek orders acquired a new function; they served to articulate or unify the building visually, organizing it into a series of coordinated visual units that could be comprehended as an integrated whole, rather than as a collection of isolated units. This concept of applied decoration was passed on through the Greco-Roman period. The triumphal arch of Rome, with its system of decorative columns and entablature articulating what is essentially one massive shape, is a particularly good illustration; the Colosseum is another. Most of the great architecture of the Renaissance and Baroque periods depends on it; to a large extent, the difference between these styles is the difference in decoration. The characteristic serenity and balance of Filippo Brunelleschi’s architecture in the 15th century, for example, is very largely effected by his treatment of pilasters (rectangular ornamental columns with bases and capitals) and entablatures applied to them, whereas, in 16th-century wall-surface designs such as Michelangelo’s Medici chapel or the dome of St. Peter’s, the same elements are used in different combinations to create a quite opposite effect of tension and release.

Judicious and intelligent use of applied ornament remained characteristic of most Western architecture until the 19th century, when the rationale of applied ornament frequently broke down, and an often indiscriminate and inappropriate use of decoration became characteristic. The reasons for this development are complex. In part it was a reaction to an overly rapid pace of social change during the period; partly, also, it was a logical outgrowth of the increasingly lavish decoration of late Baroque and Rococo architecture in the 18th century. Also, there was an overemphasis on the purely literary and associative values attached to the ornament characteristic of historical architectural styles. But compounding all these factors was the development of machinery, such as multiple lathes and jigs, which provided builders with cheap prefabricated ornament to give their often shoddy and ill-proportioned structures an illusion of elegance. Architectural ornament and architectural forms proper tended to part company and to be designed quite independently of each other.

Organic ornament

By the early 20th century a preoccupation with the proper function of architectural ornament was characteristic of many architectural thinkers, and by the mid-20th century a concept of architectural ornament had been formulated that has been called organic ornament. This concept, however, is by no means peculiar to the 20th century. Its essential principle is that ornament in architecture should derive directly from and be a function of the nature of the building and the materials used. This principle is characteristic of both Christian and Islamic religious architecture of the medieval period. In the architectural ornament of Muslim India or Persia, as in early Christian and Byzantine work, there is a strong mimetic element. The proscription of representational forms in the Qurʾān and the tendency of both Muslim and early Christian artists to borrow and adapt their formal vocabulary from preceding cultures led inevitably to their transforming what had been meaningful forms into systems of abstract ornament. But basically this ornament was neither mimetic nor applied. Throughout the Middle Ages, church buildings were conceived primarily as tangible symbols of heaven. Their architectural ornament, no matter how various or lavish, was consistently designed to promote this symbolism; whether by gilt, intricacy, or multiplicity, it all contributed to an overall effect of glory and so was integral to the architectural form.

Twentieth-century concepts of the function of architectural ornament, generally speaking, began with an understanding of this medieval usage that grew out of the 19th-century writings of the English art critic John Ruskin and the French Gothic Revival architect Eugène-Emmanuel Viollet-le-Duc, as well as through the interpretations and applications of the British designer William Morris. The immediate influence of these men proved rather unfortunate. The first result of Viollet-le-Duc’s disciplined and scholarly investigations into the principles of medieval architecture was a school of slick archaeological architects, capable of decorating all manner of collegiate, civic, and domestic buildings with frigidly correct reproductions of the details of medieval cathedrals and châteaus. Out of Ruskin’s demonstration of the origins of medieval decoration in natural forms there grew the so-called Art Nouveau movement toward exaggerated floral and curvilinear ornament, and out of Morris’s insistence on handicrafts, inspired by infatuation with the medieval guild system, developed the Arts and Crafts movement.

As early as the 1870s the U.S. architect H.H. Richardson adopted the Romanesque style, less for its historical associations than for the opportunities it afforded him to express the nature and texture of stone. In mature examples of his architecture from the mid-1880s, ornament in the older, applied sense had virtually disappeared, and his buildings depend for their aesthetic effect mainly on the inherent qualities of their materials. The generation following Richardson saw a further international development of this principle.

In Great Britain Sir Edwin Lutyens and Charles Rennie Mackintosh, in the Netherlands Hendrik Petrus Berlage, and in the United States Louis Sullivan were among many architects who contributed to the new ornamental expression. It was largely based on intrinsic texture and pattern but with interspersed bands and patches of naturalistic ornament, applied with studied discipline. With the general reaction against 19th-century eclectic principles of ornamentation after World War I, however, leading designers rejected even this kind of applied ornament and relied for ornamental effect on building materials alone. The so-called International Style, in which the German architect Walter Gropius and the Swiss-French architect Le Corbusier were the chief figures, dominated advanced design during the late 1920s and 1930s. The barrenness that resulted from their reliance on such materials as concrete and glass, however, along with other factors, resulted in a reaction in the 1940s in favour of the neglected precedent set by the U.S. architect Frank Lloyd Wright in his early 20th-century work, which emphasized more visually interesting materials, intricate textural patterns, and natural settings as the proper basis of architectural ornament. This trend continued in later decades; the style known as the New Brutalism was related to it.