Arsis

Arsis, in Prosody, originally the unaccented part of a foot; now, however, the accented portion; while thesis, now the unaccented, was formerly the accented part. In elocution arsis is the raising of the voice and thesis the depression. In music arsis is the downward beat and thesis the upward, as the ancients used to beat time in exactly the opposite way to the moderns - their upward beat signifying the accented portion of the bar.