Here, the three bars allegro-assai preceding the allegretto are beaten in simple two time, as usual. At the moment when the allegretto begins, the bar of which is double that of the preceding, and of the one maintained by the violas, the conductor marks two divided beats for the long bar, by two equal gestures down, and two others up:—
The two large gestures divide the long bar in half, and explain its value to the hautboys, without perplexing the violas, who maintain the brisk movement, on account of the little gesture which also divides in half their short bar.
From bar No. 3, the conductor ceases to divide thus the long bar by 4, on account of the triple rhythm of the melody in 6⁄8, which this gesture interferes with. He then confines himself to marking the two beats of the long bar; while the violas, already launched in their rapid rhythm, continue it without difficulty, comprehending exactly that each stroke of the conductor's stick marks merely the commencement of their short bar.
This last observation shows with what care dividing the beats of a bar should be avoided when a portion of the instruments or voices has to execute triplets upon these beats. The division, by cutting in half the second note of the triplet, renders its execution uncertain. It is even necessary to abstain from this division of the beats of a bar just before the moment when the rhythmical or melodic design is divided by three, in order not to give to the players the impression of a rhythm contrary to that which they are about to hear:—
In this example, the subdivision of the bar into six, or the division of beats into two, is useful; and offers no inconvenience during bar No. 1, when the following gesture is made:—