Arsis and thesis

In music and prosody, arsis and thesis refer to the stronger and weaker parts of a musical measure or poetic foot. Arsis and thesis were the raising and lowering of the foot in beating of time, or the raising and lowering of the voice in pitch or stress. Accordingly, in music and in Greek scansion arsis is an unaccented note (upbeat), but in Latin and modern poetry it is the stressed syllable (ictus).