In music, a standard is a musical composition of established popularity, considered part of the "standard repertoire" of one or several genres. Even though the standard repertoire of a given genre consists of a dynamic and partly subjective set of songs, these can be identified by having been performed or recorded by a variety of musical acts, often with different arrangements. In addition, standards are extensively quoted by other works and commonly serve as the basis for musical improvisation. Standards may "cross over" from one genre's repertoire to another's; for example, many jazz standards have entered the pop repertoire, and many blues standards have entered the rock repertoire.
Standards exist in the classical, popular and folk music traditions of all cultures. In the context of Western classical music, the standard repertoire constitutes most of what is considered the "teaching canon", i.e. the compositions that students learn in their academic training. The standard repertoire varies according to the different eras, movements and scenes within a genre, meaning that the extent to which a given composition is considered a standard or "repertoire piece" may vary greatly. However, some repertoires (e.g. concert piano) have become particularly static, giving rise to a divide between "standard-repertoire performers" and contemporary music advocates.
Standards mostly possess "canonical" structures which pervade the repertoire. Thus, classical piano recitals tend to contain Classical-period sonatas, as well as forms from the Baroque, Romantic and contemporary eras. Popular standards in the Western tradition often have one of four basic song structures: strophic form (AAA), twelve-bar blues progression (AAB),thirty-two-bar form (AABA) or a verse-chorus form (ABAB).