User’s Guide, Chapter 25: Post-Tonal Tools (1)

The music21 toolkit features many tools for analyzing and creating music within a post-tonal context. A Chord can be identified as a Forte class, a list of pitch classes can be used to create a 12-tone matrix, and many other analytical tools are available.

This overview will illustrate key features of music21’s post tonal tools. For complete documentation on post-tonal tools, see the many methods in Chord as well as the objects in music21.serial, such as TwelveToneMatrix and TwelveToneRow.

Pitches as Pitch Classes

Any music21 Pitch, or a Note containing Pitch, can be expressed as pitch class integers using the pitchClass and pitchClassString properties.

In the following example, the parse() function is used to create a Score object. The id attribute of each contained Part is presented in a list.

We’ll start our look at post-tonal music with a most-decidedly tonal piece, Beethoven’s Opus 59.

from music21 import *
aScore = corpus.parse('beethoven/opus59no2', 3)
[e.id for e in aScore.parts]

 ['Violin I.', 'Violin II.', 'Viola.', 'Violoncello.']

We can view the fourth and fifth measures of the violin Part by obtaining the Part from the Stream with getElementById() method. Next, we can extract the desired measures with the measures() method. Calling the show() method will, assuming correct setup of your environment, open a display of the extracted measures.

vlnPart = aScore.getElementById('Violin I.')
mRange = vlnPart.measures(4, 7)
mRange.show()

If we want to gather all Pitch objects from this measure range, we can use the pitches property. This returns a list of all Pitch objects. All pitch objects have pitchClass and pitchClassString properties, providing either integer or string representations, respectively.

', '.join([str(p) for p in mRange.pitches])

 'A4, F#4, G4, G4, B4, E5, G5, G5, G5, C#6, E6, E6, E6, G6, C#5'

[p.pitchClass for p in mRange.pitches]

 [9, 6, 7, 7, 11, 4, 7, 7, 7, 1, 4, 4, 4, 7, 1]

If we want to label the notes in our measure range with the Note’s pitch class representation, we can iterate over the notes and assign the pitch class representation to the Note’s lyric. This is a common way to annotate Note and Chord objects in music21. The results can be displayed with the show() method.

Here is the image from when we were using a Beethoven Quartet as an example:

for n in mRange.recurse().notes:
    if n.tie is None or n.tie.type == 'start':
        n.lyric = n.pitch.pitchClassString
mRange.show()

Chords as Forte Set Classes

Any music21 Chord can be interpreted as a Forte set class. Additional, a wide variety of analytical features, derived from the Forte set class, are available as methods of the chord.

For an example, lets create a sequence of generated aggregate-completing trichords stored on a Stream. That is, we will construct chords with pitch classes, drawing them from a list of all pitch classes. These pitches will be supplied to a Chord object and stored on a Stream.

aStream = stream.Stream()
src = list(range(12)) # cheate a list of integers 0 through 11
src = src[2:4] + src[0:2] + src[8:9] + src[4:8] + src[9:12] # recombine
for i in range(0, 12, 3):
    aStream.append(chord.Chord(src[i:i + 3]))

aStream.show()

These Chords, like all Chords in music21, can be interpreted as Forte set classes. The Chord object offers numerous methods that retrieve data from the set class representation of the Chord. The following is just a sampling of some of the many relevant methods.

for c in aStream:
    print(c.orderedPitchClassesString)

 <023>
 <148>
 <567>
 <9AB>

for c in aStream:
    print(c.forteClass)

 3-2B
 3-11A
 3-1
 3-1

for c in aStream:
    print(c.forteClassTnI)

 3-2
 3-11
 3-1
 3-1

for c in aStream:
    print(c.normalOrder)

 [0, 2, 3]
 [1, 4, 8]
 [5, 6, 7]
 [9, 10, 11]

for c in aStream:
    print(c.primeFormString)

 <013>
 <037>
 <012>
 <012>

for c in aStream:
    print(c.intervalVector)

 [1, 1, 1, 0, 0, 0]
 [0, 0, 1, 1, 1, 0]
 [2, 1, 0, 0, 0, 0]
 [2, 1, 0, 0, 0, 0]

To annotate the Chords stored on the Stream with their Forte name, we can iterate over the Stream and assign the Forte name to each Chord’s lyric attribute.

for c in aStream:
    c.lyric = c.forteClass
aStream.show()

We can use the itertools.combinations function to find all possible subsets of a chord for a given cardinality. (Hat tip to John Chittum for the hint). Here is the all tri-chord hexachord which contains within it all possible three-note chords under inversion and transposition:

import itertools
c = chord.Chord([0, 1, 2, 4, 7, 8])
c.commonName

 'all tri-chord hexachord'

Now we’ll test this create a set to hold the Forte number for each trichord and see that all twelve of them are there:

cc = set()
for i in itertools.combinations(c.pitches, 3):
    cc.add(chord.Chord(i).forteClassNumber)
cc

 {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12}

Creating and Processing Twelve-Tone Matrices

The music21 music21.serial module provides a Stream-based representation of a 12-Tone row, as well as the ability to view these rows as a matrix. Additionally, numerous 12-tone rows from works are available as classes.

For example, we can create an instance of the row from Alban Berg’s Violin Concerto, use the show() method to display its contents as text, and then create and print a TwelveToneMatrix object.

aRow = serial.getHistoricalRowByName('RowBergViolinConcerto')
aRow.show('text')

 {0.0} <music21.note.Note G>
 {1.0} <music21.note.Note B->
 {2.0} <music21.note.Note D>
 {3.0} <music21.note.Note F#>
 {4.0} <music21.note.Note A>
 {5.0} <music21.note.Note C>
 {6.0} <music21.note.Note E>
 {7.0} <music21.note.Note G#>
 {8.0} <music21.note.Note B>
 {9.0} <music21.note.Note C#>
 {10.0} <music21.note.Note E->
 {11.0} <music21.note.Note F>

aMatrix = aRow.matrix()
print(aMatrix)

   0  3  7  B  2  5  9  1  4  6  8  A
   9  0  4  8  B  2  6  A  1  3  5  7
   5  8  0  4  7  A  2  6  9  B  1  3
   1  4  8  0  3  6  A  2  5  7  9  B
   A  1  5  9  0  3  7  B  2  4  6  8
   7  A  2  6  9  0  4  8  B  1  3  5
   3  6  A  2  5  8  0  4  7  9  B  1
   B  2  6  A  1  4  8  0  3  5  7  9
   8  B  3  7  A  1  5  9  0  2  4  6
   6  9  1  5  8  B  3  7  A  0  2  4
   4  7  B  3  6  9  1  5  8  A  0  2
   2  5  9  1  4  7  B  3  6  8  A  0

We might divide this row into trichords, present each of those trichords as Chords, and label the resulting pitch classes and Forte set class. As shown above, we can set the lyric attribute to assign a single line of text. If we need to assign multiple lines of text, the Note and Chord method addLyric() can be used to add successive lines.

bStream = stream.Stream()
for i in range(0, 12, 3):
    c = chord.Chord(aRow[i:i + 3])
    c.addLyric(c.primeFormString)
    c.addLyric(c.forteClass)
    bStream.append(c)
bStream.show()

This should give a decent introduction to working with post-tonal music – we’ll return to this when we discuss searching for post-tonal structures, but there’s still a lot to find in a Stream, and we want to take it one step at a time – the best way to do so is to understand Chapter 26, Stream Iteration and Filtering