Chromaticism
Movement in semitone steps.
What it means
Chromaticism designates the use of notes that do not belong to the pool of notes of the prevailing key, and in particular movement in semitone steps. It stands opposed to diatonicism, which makes do with the seven basic notes of a scale. The word goes back to the Greek word for colour — the chromatic notes colour the diatonic basic stock without cancelling it.
In the notation
Chromaticism is recognizable by accidentals within the bar: sharps, flats, and naturals that do not come from the key signature. The spelling follows the direction — semitones leading upward are as a rule written with sharps, those leading downward with flats, even if both hit the same key. This distinction is not a matter of form: it shows which degree is meant and where the note goes next.
Musical use
Three applications are particularly important. First, the chromatic passing note, which connects two notes of the scale without changing the harmony. Second, alteration: a chord note is altered to heighten tension or to form a secondary dominant; in terms of functional theory, the sound remains what it was and merely receives a sharper direction. Third, modulation, in which chromatic notes produce the transition into another key.
Historically, chromaticism reaches far back: even the practice of musica ficta introduced unnotated semitones in the Middle Ages. In the madrigal of the late sixteenth century it became a means of expressing the affects, and in the nineteenth century it stretched tonality to its limits.
Chromaticism is to be distinguished from atonality: even strongly chromatic music remains tonal as long as a reference centre is audible.
In the harmony of the nineteenth century, chromaticism increases strongly; leading tones, alterations, and rapid modulations gradually dissolve the clear tie to a key.
Performance practice
With chromatic lines, watch the direction and lead them purposefully; every semitone pushes onward. On instruments with flexible intonation, it is worthwhile to take upward-leading notes close to their goal. And speak dense passages aloud once slowly with note names before you play them.