Wikis > Glossary > Anacrusis

An anacrusis, as defined by Lerdahl and Jackendoff in A Generative Theory of Tonal Music (1983), is the span from the beginning of a group to the strongest (metric) beat in a group.  Anacruses result when grouping and meter are out of phase.  Anacruses are not upbeats, because beats are not durations; an anacrusis can contain various upbeats at various levels of the metrical hierarchy.