For other uses of the term, see: Ornament
In architecture, ornament is a decorative detail used to embellish parts of a building or interior furnishing. Ornament can be carved from stone, wood or precious metals, formed with plaster or clay, or impressed onto a surface as applied ornament. A wide variety of decorative styles and motifs have been developed for architecture and the applied arts, including ceramics, furniture, metalwork and textiles.
In a 1941 essay, the architectural historian Sir John Summerson called it "surface modulation". Decoration and ornament has been evident in civilizations since the beginning of recorded history, ranging from Ancient Egyptian architecture to the apparent lack of ornament of 20th Century Modernist architecture.

Pattern Books
There were two available routes from this perceived crisis. One was to attempt to devise an ornamental vocabulary that was new and essentially contemporary. This was the route taken by architects like Louis Sullivan and his pupil Frank Lloyd Wright, or by the unique Antoni Gaudí. Art Nouveau, for all its excesses, was a conscious effort to evolve such a "natural" vocabulary of ornament.
A more radical route abandoned the use of ornament altogether, as in some designs for objects by Christopher Dresser. At the time, such unornamented objects could have been found in many unpretending workaday items of industrial design, ceramics produced at the Arabia manufactory in Finland, for instance, or the glass insulators of electric lines.
This latter approach was described by architect Adolf Loos in his 1908 manifesto, translated into English in 1913 and polemically titled Ornament and Crime, in which he declared that lack of decoration is the sign of an advanced society. His argument was that ornament is economically inefficient and "morally degenerate", and that reducing ornament was a sign of progress. Modernists were eager to point to American architect Louis Sullivan as their godfather in the cause of aesthetic simplification, dismissing the knots of intricately patterned ornament that articulated the skin of his structures.
With the work of Le Corbusier and the Bauhaus through the 1920s and 1930s, lack of decorative detail became a hallmark of modern architecture and equated with the moral virtues of honesty, simplicity, and purity. In 1932 Philip Johnson and Henry-Russell Hitchcock dubbed this the "International Style". What began as a matter of taste was transformed into an aesthetic mandate. Modernists declared their way as the only acceptable way to build. As the style hit its stride in the highly-developed postwar work of Mies van der Rohe, the tenets of 1950s modernism became so strict that even accomplished architects like Edward Durrell Stone and Eero Saarinen could be ridiculed and effectively ostracized for departing from the aesthetic rules.