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.
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:
- "In traditional music...a group of bars comprising a natural division of the melody; usually regarded as comprising two or more contrasting or complementary phrases and ending with a cadence." (Harvard Dictionary of Music, 1969)
- "A period is a structure of two consecutive phrases, often built of similar or parallel melodic material, in which the first phrase gives the impression of asking a question which is answered by the second phrase."
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."
- "the period is normatively an eight-measure structure divided into two four-measure phrases. [...] the antecedent phrase of a period begins with a two-measure basic idea. [...] measures 3-4 of the antecedent phrase bring a 'contrasting idea' that leads to a weak cadence of some kind. [...] The consequent phrase of the period repeats the antecedent but concludes with a stronger cadence. More specifically, the basic idea 'returns' in measures 5-6 and then leads to a contrasting idea, which may or may not be based on that of the antecedent." 
Sub-Saharan music and music of the African diaspora
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.
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).
- White, John D. (1976). The Analysis of Music, p.44. ISBN 0-13-033233-X.
- White (1976), p.45.
- Whittall, Arnold. "period." The Oxford Companion to Music. Oxford Music Online. Oxford University Press, accessed August 4, 2015, http://www.oxfordmusiconline.com/subscriber/article/opr/t114/e5096.
- Leonard G. Ratner. "Period." Grove Music Online. Oxford Music Online. Oxford University Press, accessed April 22, 2015, http://www.oxfordmusiconline.com/subscriber/article/grove/music/21337.
- Nattiez, Jean-Jacques (1990). Music and Discourse: Toward a Semiology of Music (Musicologie générale et sémiologue, 1987). Translated by Carolyn Abbate (1990). ISBN 0-691-02714-5.
- (1969). Harvard Dictionary of Music. Cited in Nattiez, Jean-Jacques (1990).
- White (1976), p.46.
- Caplin 'Classical Form: A Theory of Formal Functions for the Instrumental Music of Haydn, Mozart, and Beethoven' (1998), p. 12
- New Harvard Dictionary of Music (1986: 625) ed. Don Michael Randel. Cambridge: Harvard University Press.
- "The time span of the bell rhythm and its division into beats establish meter, a concept that implies a musical period" Locke, David "Improvisation in West African Musics" Music Educators Journal, Vol. 66, No. 5, (Jan., 1980), p. 125-133. Published by: MENC: The National Association for Music Education.
- Novotney, Eugene N. (1998: 165) Thesis: The 3:2 Relationship as the Foundation of Timelines in West African Musics, UnlockingClave.com. Urbana, IL: University of Illinois.
- Peñalosa, David (2012: 255) The Clave Matrix; Afro-Cuban Rhythm: Its Principles and African Origins. Redway, CA: Bembe Inc. ISBN 1-886502-80-3.
- Gerstin, Julian (2013) "Rhythmic Structures in the African Continuum" Analytical Approaches to World Music.
- Agawu, Kofi (2003: 73) Representing African Music: Postcolonial Notes, Queries, Positions. New York: Routledge.
- Nketia, Kwabena (1961: 78) African Music in Ghana. Accra: Longmans.
- Kubik, Gerhard (1999: 54) Africa and the Blues. Jackson, MI: University Press of Mississippi. ISBN 1-57806-145-8.
- “A regular and recurrent rhythm pattern played on the bell provides the time referent by which members of the performing group reckon the alignment of their rhythm patterns, song melodies, and dance movements. Not only is the basic musical period established by the bell pattern but its distinctive rhythmic shape influences all aspects of the music and dance.” Locke, David (1982: 217-218) “Principles of Off-Beat Timing and Cross-Rhythm in Southern Ewe Dance Drumming,” Society for Ethnomusicology Journal, November, p. 217.
- “Whether performed individually or shared as a collective experience, the music is nonetheless rigidly controlled by a recurrent rhythm often associated with the role of the bell pattern typical of West and Central African drumming” Anku, Wille (2000: 1) “Circles and Time: A Theory of Structural Organization of Rhythm in African Music,” Society for Music Theory 6, no. 1.
- Ladzekpo, C.K. (1995: Web) "Main Beat Schemes." Foundation Course in African Music.
- Locke, David "Agbadza: The Critical Edition" Tufts University.
- Jones, A.M. (1959: 211-212) Studies in African Music.
- Novotney (1998)
- Peñalosa (2012: 59).
- Grenet, Emilio, translated by R. Phillips (1939: XV). Popular Cuban Music New York: Bourne Inc.
- C.K. Ladzekpo considers cross-rhythm to be rhythmic conflict or an alternate motion, and a main beat concurrence to be a moment of resolution. Ladzekpo, C.K. (1995: Web) "Technique of Composite Rhythm" Foundation Course in African Music. http://home.comcast.net/~dzinyaladzekpo/PrinciplesFr.html
- Peñalosa (2012: 104).
- "[The] clave pattern has two opposing rhythm cells: the first cell consists of three strokes, or the rhythm cell, which is called 'tresillo' (Spanish tres = three). This rhythmically syncopated part of the clave is called the three-side or the strong part of the clave. The second cell has two strokes and is called the two-side or the weak part of the clave . . . The different accent types in the melodic line typically encounter with the clave strokes, which have some special name. Some of the clave strokes are accented both in more traditional tambores batá -music and in more modern salsa styles. Because of the popularity of these strokes, some special terms have been used to identify them. The second stroke of the strong part of the clave is called 'bombo'. It is the most often accented clave stroke in my research material. Accenting it clearly identifies the three-side of the clave (Peñalosa The Clave Matrix 2009, 93-94). The second common clave stroke accented among these improvisations is the third stroke of the strong part of the clave. This stroke is called 'ponche.' In Cuban popular genres, this stroke is often accented in unison breaks that transition between the song sections (Peñalosa 2009, 95; Mauleón 1993, 169)" Iivari, Ville (2011: 1, 5) The Relation Between Clave Pattern and Violin Improvisation in Santería’s Religious Feasts. Department of Musicology, University of Turku, Finland. Web. http://www.siba.fi/fi/web/embodimentofauthority/proceedings;jsessionid=07038526F10A06DE7ED190AD5B1744D7
- Amira, John and Steven Cornelius (1992: 23, 24) The Music of Santeria; Traditional Rhythms of the Batá Drums. Tempe, AZ: White Cliffs. ISBN 0-941677-24-9
- Jones (1959: 197-198)
- Gerard, Charley, and Marty Sheller (1989 :14) Salsa! The Rhythm of Latin Music. Crown Point, Indiana: White Cliffs.
- Musical Period www.artofcomposing.com