Arsis
The upbeat, the lift in the rhythmic up-and-down of the bar.
What it means
Arsis and thesis denote the pair of lift and fall out of which the rhythmic course is built. The terms come from ancient prosody and were taken over from there into music theory. Arsis means the lighter, leading-toward half, thesis the heavier, aiming one — in today’s language roughly the relation of upbeat and accented beat.
It should be noted that the assignment is not uniform in the transmission: in part of the sources arsis denotes the raising of the foot or the hand and thus the unstressed part, while in others the term is used the other way round. Whoever meets it in the literature should therefore check in which sense the particular author uses it.
Musical use
The idea behind it is more fundamental than the terminology: music runs not as a chain of equally weighted events but in motions from one point to another. Notes lead toward or fade away; a phrase has a goal. In medieval and early-modern music theory the pair of terms was bound to the beating motion of the tactus, in which the fall and the rise of the hand together formed a unit. In the chant scholarship of the nineteenth and twentieth centuries it played a role again, when the question was how the performance of liturgical chant was to be articulated — there too the terms are not uniformly used.
The meaning of arsis and thesis has been reversed in the course of history. In Greek dance theory arsis denoted the raising of the foot, that is the light part, thesis the setting-down, the heavy part. Latin prosody, by contrast, referred the words to the raising and lowering of the voice, so that arsis meant the accented part. In music theory today the original meaning is usually used: arsis is the upbeat, the light time.
Performance practice
When practising, think in goals rather than in single notes: with every group ask yourself which note carries the weight and which lead toward it. Take leading-toward notes lighter and let them press, instead of accenting them. With upbeat phrases it holds especially that the upbeat belongs to the following beat and not to the preceding bar — whoever takes it heavily cuts the line apart.