A Dictionary of Music and Musicians/Arsis and Thesis
ARSIS AND THESIS. Terms used both in music and in prosody. They are derived from the Greek. Arsis is from the verb αίρω (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 θέσισ (deposilio 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, lifting the hand for the strong beat and letting it fall for the weak, whereas we make the down beat for the strong accents, and raise our hand for the others.
When applied to the voice, a subject, counterpoint, or fugue, are said to be 'per thesin,' when the notes ascend from grave to acute; 'per arsin' when they descend from acute to grave, for here again the ancient application of the ideas of height or depth to music was apparently the reverse of our own.
A fugue 'per arsin et thesin' is the same thing as a fugue 'by inversion,' that is to say, it is a fugue in which the answer to the subject is made by contrary motion. (See Fugue, Canon, Inversion, and Subject). The terms arsis and thesis may be regarded as virtually obsolete, and are practically useless in these days.