Tonic, Mode and Key
The musical term key indicates the tonic and mode: Thus, a work in the key of C-Major has a tonic of C and is in the Major mode. A work in the key of f-minor has f for a tonic and is in the minor mode. The keys of D-Major and d-minor are called parallel keys, because they share the same tonic but are in different modes.
Staying in and Leaving a Key
Repertoires that consist largely of circular progressions tends to be rooted in a single key. By mixing both circular and linear progressions, classical music developed the possibility of changing keys.
Harmonies that remain within one key are called diatonic progressions. Moving from key to key is called modulation. Along with distinguishing between Major and minor, being able to discriminate between diatonic progressions and modulation is a cornerstone of hearing Common Practice harmony.
Classical music puts a great value on harmonic kineticism--on forward progress and suspense. Too much emphasis on the tonic would impede that: It would be like trying to drive with the emergency brake on. Thus, a classical work often avoids its main tonic for significant stretches, saving its greatest emphasis on the home key for the end.
A diatonic progression may be entirely limited to the notes of the key. The following excerpt from Georg Frederic Handel’s Water Music is consists exclusively of the pitches of its home key, D-Major.
On the other hand, modulations create novelty: They introduce new pitches and cadence in new places. This excerpt from J.S. Bach’s Partita No. 1 in B-flat Major begins in one key - g-minor - and ends in another — the home key of B-flat Major.
The treatment of themes is one of the strongest perceptual cues for modulation. Expository passages, in which themes are presented whole, generally remain within a single key. On the other hand, developmental passages, in which themes are broken into fragments, are often modulatory: Presenting the theme in its entirety would put a drag on the harmonic motion; the thematic “shorthand” allows the music to progress more rapidly. Fragmentation, along with the introduction of new pitches, is a strong indicator of modulation.
The Finale of J.S. Bach’s Concerto in d opens with the main theme presented by the string orchestra.
A later modulating passage consists only of thematic fragments.
The Finale of Brahms’ String Quintet in G-Major, Opus 111 opens with the following phrase.
A later modulating passage only uses the theme’s head motive.
Thus, themes presented whole typically remain within a key whereas fragmentation, along with introduction of new pitches, is an indication of modulation.
Classical music is loaded with modulation — that is one of its distinguishing features. Being alert to the difference between diatonic progressions and modulation invigorates your hearing of harmony and makes you more fully present to the music’s energy and drama.
We will now probe further into music that remains within a key and music that moves from key to key.