In poetry and music, and by analogy in other fields, an anacrusis (plural anacruses) is a brief introduction.
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Poetry
In poetry, a set of extrametrical syllables at the beginning of a verse is said to stand in anacrusis (Ancient Greek: ἀνάκρουσις "pushing up"). The technique is seen Old English poetry, and in lines of iambic pentameter, the technique applies a variation on the typical pentameter line causing it to appear at first glance as trochaic.
Music
In music, an anacrusis (also known as a pickup) is a note or sequence of notes which precedes the first downbeat in a bar in a musical phrase.
The musical term is inferred from the terminology of poetry, where it refers to one or more first but unstressed syllables of a lyrical verse.
The anacrusis is a perceived grouping which is context generated in the individual phrasing of a concrete composition.
The grouping of one or more antecedent tone events to a perceived phrase gestalt may be rhythmically evoked by their temporal proximity to the phrase's first downbeat (perceived phrase onset).
An anacrusis may also be evoked solely metrically (non-rhythmically ), i. e. tonally, that is, without the downbeat perception enforced by a relative long value.
Although the anacrusis is integrated in a musical phrase gestalt (grouped to it), it is not located in the perceived 'body' of the phrase (which is spanning from its first downbeat to its ending beat), but before the phrase (hence the German term "Auftakt").
In this respect -in a sequence of phrases- the anacrusis also may be perceived 'between' two phrases, neither being perceived as part of the ending of a former one, nor being located in the following one.
Outside of that the term of the anacrusis is most commonly used where it applies everywhere else 'within' the 'body' of the phrase between the 'head' (first downbeat) and the 'foot' (ending beat) where, by what ever musical means, a grouping is perceived from an upbeat to a downbeat (especially also to the phrases ending beat).
Western standards for musical notation often include the recommendation that when a piece of music begins with an anacrusis, the notation should omit a corresponding number of beats from the final bar in order to keep the length of the entire piece at a whole number of bars.
If anacrusis is present, the first bar after the anacrusis is assigned bar number 1.
Examples
Other fields
In academic publishing, the term is sometimes used in an article to mark an introductory idea standing between the abstract and the introduction proper.