Tutorial 7 - Progressions

In music theory you often deal with sequences of chords. These chord sequences are called progressions and are often written down using roman numerals. In this system the ‘I’ refers to the first natural triad in a key, the II to the second, etc. We can add prefixes and suffixes to denote more complex progressions like #V7, bIIdim7, etc.

The progressions module provides methods which can convert progressions into chords and vice versa. It can also give suggestions for chord-substitutions.

>>> import mingus.core.progressions as progressions

Functions to Chords

Remember the chord functions from the chords tutorial? As handy as they can be, they feel a bit clunky. For example: we want to take the I, IV and V7 chord in a couple of different keys:

>>> [chords.I("C"), chords.IV("C"), chords.V7("C")]
>>> [chords.I("F"), chords.IV("F"), chords.V7("F")]

As you can see, you would have to retype the actual progression everytime you needed it. Instead we can do this:

>>> progression = ["I", "IV", "V7"]
>>> progressions.to_chords(progression, "C")
>>> progressions.to_chords(progression, "F")

Which will do exactly the same thing and is generally a lot nicer and more modular.

Another advantage is that the to_chords function knows about prefixes and suffixes so you denote complex progressions. You can use any number of accidentals as prefix and any known chord shorthand as suffix:

>>> progressions.to_chords(["I", "bIV", "VIIdim7"])
[['C', 'E', 'G'], ['Fb', 'Ab', 'Cb'], ['B', 'D', 'F', 'Ab']]

Note: since the use of ‘7’ as suffix classicly means that you want the natural seventh chord instead of the natural triad, you have to use the ‘dom7’ shorthand to get the dominanth seventh - where you would use ‘7’ when talking about chords. In other words I7 will give you a major seventh, Idom7 a dominanth seventh.

Chords to Functions

Now that we can convert progressions to chords, it would be nice if we could hand mingus some chords and get the progressions back. That’s what determine is for. Here’s an example that uses the chords from the previous example:

>>> a = progressions.to_chords(["I", "bIV", "VIIdim7"])
>>> a
[['C', 'E', 'G'], ['Fb', 'Ab', 'Cb'], ['B', 'D', 'F', 'Ab']]
>>> progressions.determine(a, "C")
[['tonic'], ['minor subdominant'], ['subtonic diminished seventh']]
>>> progressions.determine(a, "C", True)
[['I'], ['bIV'], ['viidim7']]

Substitutions

substitute(progression, index, depth = 0) gives a list of possible substitutions for progression[index]. If depth > 0 the substitutions of each result will be recursively added as well.

>>> progressions.substitute(["I", "IV", "V", "I"], 0)
["III", "VI", etc.

substitute performs all kinds of substitutions. If you want more fine grained control you can use the functions substitute_harmonic, substitute_major_for_minor, substitute_minor_for_major, substitute_diminished_for_diminished and substitute_diminished_for_dominant. Check the reference section of this module to read more about them.

You can learn more about mingus.core.progressions in the reference section.