Preparation
The consonant preparation of a dissonance.
What it means
By preparation, part-writing theory understands that a note that is going to become dissonant first sounds, in the same voice, as a consonance. It is thus already present before the dissonance arises — and the dissonance results from the fact that the other voices move on while it stays put.
Musical use
Preparation, dissonance, and resolution form a fixed sequence in strict counterpoint. The note first sounds consonant on a light beat, then becomes a dissonance on a heavy beat because the harmony beneath it changes, and finally resolves stepwise, mostly downward, into a chord tone. This pattern is the basis of the suspension and, in counterpoint teaching, the subject of the fourth species, in which the upper voice is led continuously syncopated against the cantus firmus.
The point lies in the hearing: a prepared dissonance is understood as the consequence of a voice leading, not as an arbitrary sharpness. The ear already knows the note and experiences its reinterpretation, rather than being surprised by it.
Historically the rule was gradually loosened. In Renaissance vocal polyphony, preparation was mandatory for accented dissonances. The seventh of the dominant seventh chord was likewise at first prepared; already in the Baroque, however, free entry became established. In the nineteenth century, dissonances increasingly entered unprepared.
The prepared dissonance is to be distinguished from the passing note and the neighbour note, which stand unaccented and are reached stepwise, and thus need no preparation.
In strict counterpoint, preparation is obligatory for suspensions and sustained dissonances: the later dissonant note first sounds as a consonance on a light beat, then stays held while the remaining voices move on, and is finally resolved stepwise. In free composition, the dominant seventh, for instance, is often introduced without preparation.
Performance practice
Hold the prepared note deliberately while the other voices move — it should not be newly attacked. Give the dissonance weight and the resolution relaxation. And listen for the moment of reinterpretation: that is where the tension arises.