TonalityGUIDE - Tonal music basic theory for Undergraduates
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Tonal music depends on a sense of 'being in a key', and this relies on the notes and triads of the major and minor scale being heard in relation to the tonic. The hierarchical relationships that make this possible are discussed in the Introduction to Tonality.

In all but the shortest and simplest pieces, the tonic is not the same throughout. Passages within a work will be based on different scales, and the movement from one scale or key to another is called modulation. This section of TonalityGUIDE looks at the relationships between keys and how tonal music modulates from one to another.

What is a modulation?
If there is a cadence in a different key from which the music started it has - if only briefly - modulated. This very broad definition ignores how strongly the new key is established by the cadence (a perfect cadence in root position, for example, would be very strong but an interrupted cadence would be less so) and how long the music remains in that key. For a modulation to be discernible it should include two main features:

  • a perfect cadence in the new key
  • a series of three or more chords that are drawn from the diatonic scale of the new key that you are suggesting

If a piece moves quickly to a different key, a modulation that fits these criteria can be described as a passing modulation. If, however, there is then another phrase in the new key and other perfect cadences, the modulation has been confirmed or established.

The section on modulation explores how tonal music can either modulate between keys with many notes in common (said to be closely related) or those with few notes in common (said to be only distantly related). There are pages on modulations between major and minor, around the circle of fifths as well as information on ways of modulating and on spottomg modulations.

Generally speaking, the later a piece of tonal music was written, the more widely and quickly it is likely to modulate. In some music from the late-Romantic period onwards, it can be hard to determine the key of quite long passages. Many commentators see in this the beginnings of early twentieth century non-tonal styles.