In music theory, the word inversion has several meanings. There are inverted chords, inverted melodies, inverted intervals, and (in counterpoint) inverted voices. The concept of inversion also plays a role in musical set theory.

Inverted chords

A chord's inversion describes the relationship of its bass to the other tones in the chord. For instance, a C major triad contains the tones C, E and G; its inversion is determined by which of these tones is used as the bottom note in the chord.

The term inversion is often used to categorically refer to the different possibilities, although it may also be restricted to only those chords where the bass note is not also the root of the chord (see root position below). In texts that make this restriction, the term position may be used instead to refer to all of the possibilities as a category.

Root position

A root-position chord is sometimes known as the parent chord of its inversions. For example, C is the root of a C major triad and is in the bass when the triad is in root position; the 3rd and the 5th of the triad are sounded above the bass. Thus, a root-position chord is also known as a $\{\}^5\_3$ chord.

The following chord is also a C major triad in root position, since the root is still in the bass. The rearrangement of the notes above the bass into different octaves (here, the note E) and the doubling of notes (here, G), is known as voicing.

Inversions

In an inverted chord, the root is not in the bass (i.e., is not the lowest note). The inversions are numbered in the order their bass tones would appear in a closed root position chord (from bottom to top).

In the first inversion of a C major triad, the bass is E—the 3rd of the triad—with the 5th and the root stacked above it (the root now shifted an octave higher), forming the intervals of a 3rd and a 6th above the inverted bass of E, respectively. A first-inversion triad is also known as a $\{\}^6\_3$ chord.

In the second inversion, the bass is G—the 5th of the triad—with the root and the 3rd above it (both again shifted an octave higher), forming a 4th and a 6th above the (inverted) bass of G, respectively. A second-inversion triad is also known as a $\{\}^6\_4$ chord. This inversion can be either consonant or dissonant, and analytical notation sometimes treats it differently depending on the harmonic and voice-leading context in which it occurs (e.g. see The cadential six-four chord below).

Third inversions exist only for chords of four or more tones, such as 7th chords. In a third-inversion chord, the 7th of the chord is in the bass position. For example, a C major 7th chord in third inversion consists of B in the bass position, with C, E and G above it— being intervals of a 2nd, 4th and 6th above the (inverted) bass of B, respectively.

Notating root position and inversions

Figured bass

In figured bass, Arabic numerals (figures) are written below each bass note. These figures refer to intervals above the bass (usually assuming octave equivalence). In a root-position triad, the intervals above the root are a 5th and a 3rd, giving the figures $\{\}^5\_3$. Normally, however, this is abbreviated by assuming that any bass note given without symbols indicates a $\{\}^5\_3$ chord by default. Similarly, the full figuring of the first inversion ($\{\}^6\_3$ ) is abbreviated to just $\{\}^6\_\{\}$; the full figuring of the second inversion ($\{\}^6\_4$ ) has no abbreviation.

Figured bass is also applied to 7th chords, which have four tones. A root-position dominant-7th chord contains a 7th, 5th, and 3rd. The full figuring of 7 5 3 is usually abbreviated to just $\{\}^7\_\{\}$; the full figuring of the first inversion (6 5 3) is usually rendered as just $\{\}^6\_5$, the second inversion (6 4 3) as $\{\}^4\_3$, and the third inversion (6 4 2) as $\{\}^4\_2$.

The figures are often used on their own (without the bass) in music theory simply to specify a chord's inversion. This is the basis for the terms given above such as "$\{\}^6\_4$ chord"; similarly, in harmonic analysis the term $I\{\}^6\_\{\}$ refers to a tonic triad in first inversion.

Popular-music notation

A notation for chord inversion often used in popular music is to write the name of a chord followed by a forward slash and then the name of the bass note. For example, the C chord above, in first inversion (i.e., with E in the bass) may be notated as C/E. This notation works even when a note not present in a triad is the bass; for example, F/G is a way of notating a particular approach to voicing a G11th chord (G–F–A–C). (This is quite different from analytical notations of function; e.g., the use of IV/V or S/D to represent the subdominant of the dominant).

Lower-case letters

Lower-case letters may be placed after a chord symbol to indicate root position or inversion. Hence, in the key of C major, the C major chord below in first inversion may be notated as Ib, indicating chord I, first inversion. (Less commonly, the root of the chord is named, followed by a lower-case letter: Cb). If no letter is added, the chord is assumed to be in root inversion, as though a had been inserted.

Arabic numerals

A less common notation is to place the number 1, 2 or 3 etc. after a chord to indicate that it is in first, second, or third inversion respectively. The C chord above in root position is notated as C, and in first inversion as C1. (This notation is quite different from the Arabic numerals placed after note names to indicate the octave of a tone, typically used in acoustical contexts; for example, C4 is often used to mean the single tone middle C, and C3 the tone an octave below it.)

Cadential six-four chord

The cadential $\{\}^6\_4$ (Figure 3) is a common harmonic phenomenon that is analyzed in two different ways: the first labels it as a second-inversion chord; the second treats it instead as part of a horizontal progression involving voice leading above a stationary bass.

1. In the first option, the cadential $\{\}^6\_4$ chord is considered a second inversion tonic triad because of the tones it contains. Under this designation, the progression is labeled: $I\{\}^6\_4,\; V,\; I$. Unlike the alternative analysis (see below), this label does not indicate any difference between a cadential $\{\}^6\_4$ and other uses of $\{\}^6\_4$ chords. Most older harmony textbooks use this label, and it can be traced back to the early 19th century.
2. In the second option, this chord is not considered an inversion of a tonic triad but as a dissonance resolving to a consonant dominant harmony. This is notated as $V\{\}^\{6-5\}\_\{4-3\},\; I$, in which the $\{\}^6\_4$ is not the inversion of the $V\{\}^\{\}\_\{\}$ chord, but a dissonance that resolves to $V\{\}^5\_3$ (that is, $V\{\}^\{5-5\}\_\{4-3\},\; V$). This function is very similar to the resolution of a 4–3 suspension. Several modern textbooks prefer this conception of the cadential $\{\}^6\_4$, which can also be traced back to the early 19th century.

Inverted intervals

An interval is inverted by raising or lowering either of the notes the necessary number of octaves, so that both retain their names (pitch class) and the one which was higher is now lower and vice versa, changing the perspective or relation between the pitch classes. For example, the inversion of an interval consisting of a C with an E above it is an E with a C above it - to work this out, the C may be moved up, the E may be lowered, or both may be moved.

Under inversion, perfect intervals remain perfect, major intervals become minor and the reverse, augmented intervals become diminished and the reverse. (Double diminished intervals become double augmented intervals, and the reverse.) Traditional interval names sum to nine: seconds become sevenths and the reverse, thirds become sixes and the reverse, and fourths become fifths and the reverse. Thus a perfect fourth becomes a perfect fifth, an augmented fourth becomes a diminished fifth, and a simple interval (that is, one that is narrower than an octave) and its inversion, when added together, will equal an octave. See also complement (music).

Counterpoint

Contrapuntal inversion requires that two melodies, having accompanied each other once, do it again with the melody that had been in the high voice now in the low, and vice versa. Also called "double counterpoint" (if two voices are involved) or "triple counterpoint" (if three), themes that can be developed in this way are said to involve themselves in "invertible counterpoint." The action of changing the voices is called "textural inversion".

Invertible counterpoint can occur at various intervals, usually the octave (8va), less often at the 10th or 12th. To calculate the interval of inversion, add the intervals by which each voice has moved and subtract one. For example: If motive A in the high voice moves down a 6th, and motive B in the low voice moves up a 5th, in such a way as to result in A and B having exchanged registers, then the two are in double counterpoint at the 10th ((6+5)–1 = 10).

Invertible counterpoint achieves its highest expression in the four canons of JS Bach's Art of Fugue, with the first canon at the octave, the second canon at the 10th, the third canon at the 12th, and the fourth canon in augmentation and contrary motion. Other exemplars can be found in the fugues in G minor and B-flat major [external Shockwave movies] from Book II of Bach's Well-Tempered Clavier, both of which contain invertible counterpoint at the octave, 10th, and 12th.

Inverted melodies

When applied to melodies, the inversion of a given melody is the melody turned upside-down. For instance, if the original melody has a rising major third (see interval), the inverted melody has a falling major third (or perhaps more likely, in tonal music, a falling minor third, or even some other falling interval). Similarly, in twelve-tone technique, the inversion of the tone row is the so-called prime series turned upside-down.

Inversional equivalency

Inversional equivalency or inversional symmetry is the concept that intervals, chords, and other sets of pitches are the same when inverted. It is similar to enharmonic equivalency and octave equivalency and even transpositional equivalency. Inversional equivalency is used little in tonal theory, though it is assumed a set which may be inverted onto another are remotely in common. However, taking them to be identical or near-identical is only assumed in musical set theory.

All sets of pitches with inversional symmetry have a center or axis of inversion. For example, the set C–E–F–F♯–G–B has one center at the dyad F and F♯ and another at the tritone, B/C, if listed F♯–G–B–C–E–F. For C–E♭–E–F♯–G–B♭ the center is F and B if listed F♯–G–B♭–C–E♭–E. (Wilson 1992, p.10-11)

Musical set theory

In musical set theory inversion may be usefully thought of as the compound operation transpositional inversion, which is the same sense of inversion as in the Inverted melodies section above, with transposition carried out after inversion. Pitch inversion by an ordered pitch interval may be defined as:

• $T^p\_nI(x)\; =\; -x+n$

which equals

• $T^p\_nI(x)\; =\; n-x$

First invert the pitch or pitches, x = −x, then transpose, −x + n.

Pitch class inversion by a pitch class interval may be defined as:

• $T\_nI(x)=-x+n\; (mod\; 12)$

History

In the theories of Rameau (1722), chords in different positions were considered functionally equivalent. However, theories of counterpoint before Rameau spoke of different intervals in different ways, such as the regola delle terze e seste ("rule of sixths and thirds") which required the resolution of imperfect consonances to perfect ones, and would not propose a similarity between $\{\}^6\_4$ and $\{\}^5\_3$ sonorities, for instance.

See also

• Voicing (music)

References

• Wilson, Paul (1992). The Music of Béla Bartók. ISBN 0-300-05111-5.