ARSIS and THESIS. Terms used both in music and in prosody. They are derived from the Greek. Arsis is from the verb ɑἴρɷ (tollo, I lift or raise), and marks the elevation of the voice in singing, or the hand in beating time. The depression which follows it is called θέσιs (depositio or remissio).
When applied to beating time, arsis indicates the strong beat, and thesis the weak: for the ancients beat time in exactly the reverse way to ours,...