There is another dilemma, extremely troublesome for a conductor, and demanding all his presence of mind. It is that presented by the super-addition of different bars. It is easy to conduct a bar in dual time placed above or beneath another bar in triple time, if both have the same kind of movement. Their chief divisions are then equal in duration, and one needs only to divide them in half, marking the two principal beats:—
But if, in the middle of a piece slow in movement, there is introduced a new form brisk in movement, and if the composer (either for the sake of facilitating the execution of the quick movement, or because it was impossible to write otherwise) has adopted for this new movement the short bar which corresponds with it, there may then occur two, or even three short bars super-added to a slow bar:—
Bar No. 1.
Bars Nos. 2, 3, and so on.
The conductor's task is to guide and keep together these different bars of unequal number and dissimilar movement. He attains this by dividing the beats in the andante bar, No. 1, which precedes the entrance of the allegro in 6⁄8, and by continuing to divide them; but taking care to mark the division more decidedly. The players of the allegro in 6⁄8 then comprehend that the two gestures of the conductor represent the two beats of their short bar, while the players of the andante take these same gestures merely for a divided beat of their long bar.
It will be seen that this is really quite simple, because the division of the short bar, and the subdivisions of the long one, mutually correspond. The following example, where a slow bar is super-added to the short ones, without this correspondence existing, is more awkward:—