In classical poetry and music, the stressed or accented part of a metrical foot, or the upbeat in musical notation.
From Greek ársis (lifting, raising). In ancient Greek poetry, arsis literally referred to the lifting of the foot while reciting, contrasting with thesis (the lowering), metaphorically representing stress.
The concept of arsis and thesis flipped meanings during the Middle Ages—what was 'lifting' in Greek became 'downbeat' in modern music, which is why musicians today use it differently than ancient poets did!
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