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In music, period refers to certain types of recurrence in small-scale formal structure. In twentieth-century music scholarship, the term is usually used as defined by the Oxford Companion to Music: "a period consists of two phrases, antecedent and consequent, each of which begins with the same basic motif." Earlier usage varied somewhat, but usually referred to a similar notions of symmetry, recurrence, and closure. The concept of a musical period originates in comparisons between music structure and rhetoric at least as early as the 16th century.
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Western art music
In Western art music or Classical music, a period is a group of phrases consisting usually of at least one antecedent phrase and one consequent phrase totaling about 8 measures in length (though this varies depending on meter and tempo). Generally, the antecedent ends in a weaker and the consequent in a stronger cadence; often, the antecedent ends in a half cadence while the consequent ends in an authentic cadence. Frequently, the consequent strongly parallels the antecedent, even sharing most of the material save the final measures. In other cases, the consequent may differ greatly (for example, the period in the beginning of the second movement of the Pathetique Sonata).
The 1958 Encyclopédie Fasquelle defines a period as follows:
Another definition is as follows:
And
A double period is, "a group of at least four phrases...in which the first two phrases form the antecedent and the third and fourth phrases together form the consequent."
When analyzing Classical music, contemporary music theorists usually employ a more specific formal definition, such as the following by William Caplin:
Bell patterns
The second definition of period in the New Harvard Dictionary of Music states: "A musical element that is in some way repeated," applying "to the units of any parameter of music that embody repetitions at any level." In some sub-Saharan music and music of the African diaspora, the bell pattern embodies this definition of period. The bell pattern (also known as a key pattern, guide pattern, phrasing referent, timeline, or asymmetrical timeline) is repeated throughout the entire piece, and is the principal unit of musical time and rhythmic structure by which all other elements are arranged. The period is often a single measure (four main beats).
The seven-stroke standard bell pattern is one of the most commonly used representations of the musical period in sub-Saharan music. The first three strokes of the bell are antecedent, and the remaining four strokes are consequent. The consequent diametrically opposes the antecedent.
Clave
Cuban musicologist Emilio Grenet represents the period in two measures of 2/4. In explaining the structure of music guided by the five-stroke African bell pattern known in Cuba as clave (Spanish for 'key' or 'code'), Grenet uses what could be considered a definition of period: "We find that all its melodic design is constructed on a rhythmic pattern of two measures, as though both were only one, the first is antecedent, strong, and the second is consequent, weak."
As Grenet and many others describe the period, the cross-rhythmic antecedent ('tresillo') is strong and the on-beat resolution is weak. This is the opposite of Western harmonic theory, where resolution is described as strong. Despite this difference, both the harmonic and rhythmic periods have consequent resolution. In simplest terms, that resolution occurs harmonically when the tonic is sounded, and in clave-based rhythm when the last main beat is sounded. Metric consonance is achieved when the last stroke of clave coincides with the last main beat (last quarter note) of the consequent measure.
The antecedent measure has three strokes and is called the three-side of clave. The consequent measure has two strokes and is called the two-side. The three-side gives the impression of asking a question, which is answered by the two-side. The two sides of clave cycle in a type of repeating call and response.
[With] clave . . . the two measures are not at odds, but rather, they are balanced opposites like positive and negative, expansive and contractive or the poles of a magnet. As the pattern is repeated, an alternation from one polarity to the other takes place creating pulse and rhythmic drive. Were the pattern to be suddenly reversed, the rhythm would be destroyed as in a reversing of one magnet within a series . . . the patterns are held in place according to both the internal relationships between the drums and their relationship with clave . . . Should the drums fall out of clave (and in contemporary practice they sometimes do) the internal momentum of the rhythm will be dissipated and perhaps even broken—Amira and Cornelius (1992).
An actual key pattern does not need to be played in order for a key pattern to define the period.