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Triads

In music theory, scale degrees are the most common way of describing and analyzing consecutive tonal harmonies; two identical chords/chord progressions are written in the same way regardless of the key. The labelling is based on triads built on the notes of a scale and numbered I … VII, counting from the tonic. The most common triads in any key are the tonic (I) and dominant (V).

This labelling system was introduced by Joseph Vogler (1749–1814). Major chords are sometimes labelled with Roman numerals and minor chords with symbols (i, ii, iii, and so on). This system was introduced by Gottfried Weber (1779–1839). The system has not become a general standard, but the different versions are similar enough to be understood irrespective of the version in use.

The notes of each triad can be seen to represent a certain degree of a key. A tonic chord includes the 1st, 3rd, and 5th degrees, the most stable degrees of a key. On the other hand, the VII chord includes the 2nd, 4th, and 7th degrees, the most active degrees of a key. It is common to see a chord as a structure of thirds. The superimposed thirds can be major (M3) or minor (m3), which gives us triads of four types:

M3 + m3 = major chord
m3 + M3 = minor chord
m3 + m3 = diminished triad
M3 + M3 = augmented triad

The following example shows us triads in C major and C minor. The natural minor scale III in a minor key is the same as the tonic in a relative major, so minor keys usually include chords accordant with the harmonic minor scale, of which the V and VII (in a box) are the most common. The note leading to the tonic establishes a stable sense of key.

A natural minor scale includes the notes defined by the key signature.

In a harmonic minor scale, the leading note or the seventh degree as to the key signature is raised.

A melodic minor scale includes a leading note and a raised sixth degree when moving upwards. A melodic minor scale downwards is like the natural minor scale.

Since the sixth and seventh degree of a minor key may in one composition conform to any of the minor scales, defining indexes are sometimes necessary. The degree sign (°) symbolizes a diminished chord. The plus sign (+) symbolises an augmented chord. Numerical and prefix symbols based on the figured bass labelling are also quite common.

Major scale triads

DExample

Natural minor scale triads

Example

Common structures in harmonic and melodic minor scales

Example

Scale degrees in a major key are more fixed than those in a minor key, where factors concerned with melody and voice leading determine the "colouring" used for the sixth and seventh degrees. Even the scale degree labellings for melodies in a minor key vary. There are several different ways of writing the scale degree symbols for a melodic minor. The method introduced above is simple but unfortunately ambiguous for some parts.

From the viewpoint of music analysis, it is not important that the symbols used represent a certain chord unambiguously; what is essential is that several chords in a minor key are ambiguous. The III chord in a minor key is the same as the tonic chord in a relative major key. If it is preceded by the VII degree, and if that is preceded by the IV degree, the progression can be perceived as a II-V-I progression in a relative major key, which it actually is in the final analysis (see the following example).

The symbol for a second inversion of the tonic chord does not describe chord function in a cadence, so it is recommended that the notation applied in bar six is used. The same chord can appear in a progression with passing notes, as in bar five; this justifies the symbol used.

Example

The symbol for a second inversion of the tonic chord does not describe chord function in a cadence, so it is recommended that the notation applied in bar six is used. The same chord can appear in a progression with passing notes, as in bar five; this justifies the symbol used.

Example