music21.figuredBass.possibility

A possibility is a tuple with pitches, and is intended to encapsulate a possible solution to a Segment. Unlike a Chord, the ordering of a possibility does matter. The assumption throughout fbRealizer is that a possibility is always in order from the highest part to the lowest part, and the last element of each possibility is the bass.

Note

fbRealizer supports voice crossing, so the order of pitches from lowest to highest may not correspond to the ordering of parts.

Here, a possibility is created. G5 is in the highest part, and C4 is the bass. The highest part contains the highest Pitch, and the lowest part contains the lowest Pitch. No voice crossing is present.

>>> G5 = pitch.Pitch('G5')
>>> C5 = pitch.Pitch('C5')
>>> E4 = pitch.Pitch('E4')
>>> C4 = pitch.Pitch('C4')
>>> p1 = (G5, C5, E4, C4)

Here, another possibility is created with the same pitches, but this time, with voice crossing present. C5 is in the highest part, but the highest Pitch G5 is in the second highest part.

>>> p2 = (C5, G5, E4, C4)

The methods in this module are applied to possibilities, and fall into three main categories:

1) Single Possibility Methods. These methods are applied in finding correct possibilities in allCorrectSinglePossibilities().

2) Consecutive Possibility Methods. These methods are applied to (possibA, possibB) pairs in allCorrectConsecutivePossibilities(), possibA being any correct possibility in segmentA and possibB being any correct possibility in segmentB.

3) Special Resolution Methods. These methods are applied in allCorrectConsecutivePossibilities() as applicable if the pitch names of a Segment correctly spell out an augmented sixth, dominant seventh, or diminished seventh chord. They are located in resolution.

The application of these methods is controlled by corresponding instance variables in a Rules object provided to a Segment.

Note

The number of parts and maxPitch are universal for a FiguredBassLine.

Functions

music21.figuredBass.possibility.voiceCrossing(possibA)

Returns True if there is voice crossing present between any two parts in possibA. The parts from the lowest part to the highest part (right to left) must correspond to increasingly higher pitches in order for there to be no voice crossing. Comparisons between pitches are done using pitch comparison methods, which are based on pitch space values (see Pitch).

>>> from music21.figuredBass import possibility
>>> C4 = pitch.Pitch('C4')
>>> E4 = pitch.Pitch('E4')
>>> C5 = pitch.Pitch('C5')
>>> G5 = pitch.Pitch('G5')
>>> possibA1 = (C5, G5, E4)
>>> possibility.voiceCrossing(possibA1)  # G5 > C5
True
>>> possibA2 = (C5, E4, C4)
>>> possibility.voiceCrossing(possibA2)
False

music21.figuredBass.possibility.isIncomplete(possibA, pitchNamesToContain)

Returns True if possibA is incomplete, if it doesn’t contain at least one of every pitch name in pitchNamesToContain. For a Segment, pitchNamesToContain is pitchNamesInChord.

If possibA contains excessive pitch names, a PossibilityException is raised, although this is not a concern with the current implementation of fbRealizer.

>>> from music21.figuredBass import possibility
>>> C3 = pitch.Pitch('C3')
>>> E4 = pitch.Pitch('E4')
>>> G4 = pitch.Pitch('G4')
>>> C5 = pitch.Pitch('C5')
>>> Bb5 = pitch.Pitch('B-5')
>>> possibA1 = (C5, G4, E4, C3)
>>> pitchNamesA1 = ['C', 'E', 'G', 'B-']
>>> possibility.isIncomplete(possibA1, pitchNamesA1)  # Missing B-
True
>>> pitchNamesA2 = ['C', 'E', 'G']
>>> possibility.isIncomplete(possibA1, pitchNamesA2)
False

music21.figuredBass.possibility.upperPartsWithinLimit(possibA, maxSemitoneSeparation=12)

Returns True if the pitches in the upper parts of possibA are found within maxSemitoneSeparation of each other. The upper parts of possibA are all the pitches except the last.

The default value of maxSemitoneSeparation is 12 semitones, enharmonically equivalent to a perfect octave. If this method returns True for this default value, then all the notes in the upper parts can be played by most adult pianists using just the right hand.

>>> from music21.figuredBass import possibility
>>> C3 = pitch.Pitch('C3')
>>> E3 = pitch.Pitch('E3')
>>> E4 = pitch.Pitch('E4')
>>> G4 = pitch.Pitch('G4')
>>> C5 = pitch.Pitch('C5')
>>> possibA1 = (C5, G4, E4, C3)
>>> possibility.upperPartsWithinLimit(possibA1)
True

Here, C5 and E3 are separated by almost two octaves.

>>> possibA2 = (C5, G4, E3, C3)
>>> possibility.upperPartsWithinLimit(possibA2)
False

music21.figuredBass.possibility.pitchesWithinLimit(possibA, maxPitch=<music21.pitch.Pitch B5>)

Returns True if all pitches in possibA are less than or equal to the maxPitch provided. Comparisons between pitches are done using pitch comparison methods, which are based on pitch space values (see Pitch).

Used in Segment to filter resolutions of special Segments which can have pitches exceeding the universal maxPitch of a FiguredBassLine.

>>> from music21.figuredBass import possibility
>>> from music21.figuredBass import resolution
>>> G2 = pitch.Pitch('G2')
>>> D4 = pitch.Pitch('D4')
>>> F5 = pitch.Pitch('F5')
>>> B5 = pitch.Pitch('B5')
>>> domPossib = (B5, F5, D4, G2)
>>> possibility.pitchesWithinLimit(domPossib)
True
>>> resPossib = resolution.dominantSeventhToMajorTonic(domPossib)
>>> resPossib  # Contains C6 > B5
(<music21.pitch.Pitch C6>,
 <music21.pitch.Pitch E5>,
 <music21.pitch.Pitch C4>,
 <music21.pitch.Pitch C3>)
>>> possibility.pitchesWithinLimit(resPossib)
False

music21.figuredBass.possibility.partPairs(possibA, possibB)

Groups together pitches of possibA and possibB which correspond to the same part, constituting a shared part.

>>> from music21.figuredBass import possibility
>>> C4 = pitch.Pitch('C4')
>>> D4 = pitch.Pitch('D4')
>>> E4 = pitch.Pitch('E4')
>>> F4 = pitch.Pitch('F4')
>>> G4 = pitch.Pitch('G4')
>>> B4 = pitch.Pitch('B4')
>>> C5 = pitch.Pitch('C5')
>>> possibA1 = (C5, G4, E4, C4)
>>> possibB1 = (B4, F4, D4, D4)
>>> possibility.partPairs(possibA1, possibA1)
[(<music21.pitch.Pitch C5>, <music21.pitch.Pitch C5>),
 (<music21.pitch.Pitch G4>, <music21.pitch.Pitch G4>),
 (<music21.pitch.Pitch E4>, <music21.pitch.Pitch E4>),
 (<music21.pitch.Pitch C4>, <music21.pitch.Pitch C4>)]
>>> possibility.partPairs(possibA1, possibB1)
[(<music21.pitch.Pitch C5>, <music21.pitch.Pitch B4>),
 (<music21.pitch.Pitch G4>, <music21.pitch.Pitch F4>),
 (<music21.pitch.Pitch E4>, <music21.pitch.Pitch D4>),
 (<music21.pitch.Pitch C4>, <music21.pitch.Pitch D4>)]

music21.figuredBass.possibility.parallelFifths(possibA, possibB)

Returns True if there are parallel fifths between any two shared parts of possibA and possibB.

If pitchA1 and pitchA2 in possibA are separated by a simple interval of a perfect fifth, and they move to a pitchB1 and pitchB2 in possibB also separated by the simple interval of a perfect fifth, then this constitutes parallel fifths between these two parts.

If the method returns False, then no two shared parts have parallel fifths. The method returns True as soon as two shared parts with parallel fifths are found.

>>> from music21.figuredBass import possibility
>>> C3 = pitch.Pitch('C3')
>>> D3 = pitch.Pitch('D3')
>>> G3 = pitch.Pitch('G3')
>>> A3 = pitch.Pitch('A3')
>>> A4 = pitch.Pitch('A4')
>>> B4 = pitch.Pitch('B4')

Here, the bass moves from C3 to D3 and the tenor moves from G3 to A3. The interval between C3 and G3, as well as between D3 and A3, is a perfect fifth. These two parts, and therefore the two possibilities, have parallel fifths.

>>> possibA1 = (B4, G3, C3)
>>> possibB1 = (A4, A3, D3)
>>> possibility.parallelFifths(possibA1, possibB1)
True

Now, the tenor moves instead to F3. The interval between D3 and F3 is a minor third. The bass and tenor parts don’t form parallel fifths. The soprano part forms parallel fifths with neither the bass nor tenor parts. The two possibilities, therefore, have no parallel fifths.

>>> F3 = pitch.Pitch('F3')
>>> possibA2 = (B4, G3, C3)
>>> possibB2 = (A4, F3, D3)
>>> possibility.parallelFifths(possibA2, possibB2)
False

music21.figuredBass.possibility.parallelOctaves(possibA, possibB)

Returns True if there are parallel octaves between any two shared parts of possibA and possibB.

If pitchA1 and pitchA2 in possibA are separated by a simple interval of a perfect octave, and they move to a pitchB1 and pitchB2 in possibB also separated by the simple interval of a perfect octave, then this constitutes parallel octaves between these two parts.

If the method returns False, then no two shared parts have parallel octaves. The method returns True as soon as two shared parts with parallel octaves are found.

>>> from music21.figuredBass import possibility
>>> C3 = pitch.Pitch('C3')
>>> D3 = pitch.Pitch('D3')
>>> G3 = pitch.Pitch('G3')
>>> A3 = pitch.Pitch('A3')
>>> C4 = pitch.Pitch('C4')
>>> D4 = pitch.Pitch('D4')

Here, the soprano moves from C4 to D4 and the bass moves from C3 to D3. The interval between C3 and C4, as well as between D3 and D4, is a parallel octave. The two parts, and therefore the two possibilities, have parallel octaves.

>>> possibA1 = (C4, G3, C3)
>>> possibB1 = (D4, A3, D3)
>>> possibility.parallelOctaves(possibA1, possibB1)
True

Now, the soprano moves down to B3. The interval between D3 and B3 is a major sixth. The soprano and bass parts no longer have parallel octaves. The tenor part forms a parallel octave with neither the bass nor soprano, so the two possibilities do not have parallel octaves. (Notice, however, the parallel fifth between the bass and tenor!)

>>> B3 = pitch.Pitch('B3')
>>> possibA2 = (C4, G3, C3)
>>> possibB2 = (B3, A3, D3)
>>> possibility.parallelOctaves(possibA2, possibB2)
False

music21.figuredBass.possibility.hiddenFifth(possibA, possibB)

Returns True if there is a hidden fifth between shared outer parts of possibA and possibB. The outer parts here are the first and last elements of each possibility.

If sopranoPitchA and bassPitchA in possibA move to a sopranoPitchB and bassPitchB in possibB in similar motion, and the simple interval between sopranoPitchB and bassPitchB is that of a perfect fifth, then this constitutes a hidden octave between the two possibilities.

>>> from music21.figuredBass import possibility
>>> C3 = pitch.Pitch('C3')
>>> D3 = pitch.Pitch('D3')
>>> E3 = pitch.Pitch('E3')
>>> F3 = pitch.Pitch('F3')
>>> E5 = pitch.Pitch('E5')
>>> A5 = pitch.Pitch('A5')

Here, the bass part moves up from C3 to D3 and the soprano part moves up from E5 to A5. The simple interval between D3 and A5 is a perfect fifth. Therefore, there is a hidden fifth between the two possibilities.

>>> possibA1 = (E5, E3, C3)
>>> possibB1 = (A5, F3, D3)
>>> possibility.hiddenFifth(possibA1, possibB1)
True

Here, the soprano and bass parts also move in similar motion, but the simple interval between D3 and Ab5 is a diminished fifth. Consequently, there is no hidden fifth.

>>> Ab5 = pitch.Pitch('A-5')
>>> possibA2 = (E5, E3, C3)
>>> possibB2 = (Ab5, F3, D3)
>>> possibility.hiddenFifth(possibA2, possibB2)
False

Now, we have the soprano and bass parts again moving to A5 and D3, whose simple interval is a perfect fifth. However, the bass moves up while the soprano moves down. Therefore, there is no hidden fifth.

>>> E6 = pitch.Pitch('E6')
>>> possibA3 = (E6, E3, C3)
>>> possibB3 = (A5, F3, D3)
>>> possibility.hiddenFifth(possibA3, possibB3)
False

music21.figuredBass.possibility.hiddenOctave(possibA, possibB)

Returns True if there is a hidden octave between shared outer parts of possibA and possibB. The outer parts here are the first and last elements of each possibility.

If sopranoPitchA and bassPitchA in possibA move to a sopranoPitchB and bassPitchB in possibB in similar motion, and the simple interval between sopranoPitchB and bassPitchB is that of a perfect octave, then this constitutes a hidden octave between the two possibilities.

>>> from music21.figuredBass import possibility
>>> C3 = pitch.Pitch('C3')
>>> D3 = pitch.Pitch('D3')
>>> E3 = pitch.Pitch('E3')
>>> F3 = pitch.Pitch('F3')
>>> A5 = pitch.Pitch('A5')
>>> D6 = pitch.Pitch('D6')

Here, the bass part moves up from C3 to D3 and the soprano part moves up from A5 to D6. The simple interval between D3 and D6 is a perfect octave. Therefore, there is a hidden octave between the two possibilities.

>>> possibA1 = (A5, E3, C3)
>>> possibB1 = (D6, F3, D3)  # Perfect octave between soprano and bass.
>>> possibility.hiddenOctave(possibA1, possibB1)
True

Here, the bass part moves up from C3 to D3 but the soprano part moves down from A6 to D6. There is no hidden octave since the parts move in contrary motion.

>>> A6 = pitch.Pitch('A6')
>>> possibA2 = (A6, E3, C3)
>>> possibB2 = (D6, F3, D3)
>>> possibility.hiddenOctave(possibA2, possibB2)
False

music21.figuredBass.possibility.voiceOverlap(possibA, possibB)

Returns True if there is voice overlap between any two shared parts of possibA and possibB.

Voice overlap can occur in two ways:

1) If a pitch in a lower part in possibB is higher than a pitch in a higher part in possibA. This case is demonstrated below.

2) If a pitch in a higher part in possibB is lower than a pitch in a lower part in possibA.

In the above example, possibA has G4 in the bass and B4 in the soprano. If the bass moves up to C5 in possibB, that would constitute voice overlap because the bass in possibB would be higher than the soprano in possibA.

>>> from music21.figuredBass import possibility
>>> C4 = pitch.Pitch('C4')
>>> D4 = pitch.Pitch('D4')
>>> E4 = pitch.Pitch('E4')
>>> F4 = pitch.Pitch('F4')
>>> G4 = pitch.Pitch('G4')
>>> C5 = pitch.Pitch('C5')

Here, case #2 is demonstrated. There is overlap between the soprano and alto parts, because F4 in the soprano in possibB1 is lower than the G4 in the alto in possibA1. Note that neither possibility has to have voice crossing for voice overlap to occur, as shown.

>>> possibA1 = (C5, G4, E4, C4)
>>> possibB1 = (F4, F4, D4, D4)
>>> possibility.voiceOverlap(possibA1, possibB1)
True
>>> possibility.voiceCrossing(possibA1)
False
>>> possibility.voiceCrossing(possibB1)
False

Here is the same example as above, except the soprano of the second possibility is now B4, which does not overlap the G4 of the first. Now, there is no voice overlap.

>>> B4 = pitch.Pitch('B4')
>>> possibA2 = (C5, G4, E4, C4)
>>> possibB2 = (B4, F4, D4, D4)
>>> possibility.voiceOverlap(possibA2, possibB2)
False

music21.figuredBass.possibility.partMovementsWithinLimits(possibA, possibB, partMovementLimits=None)

Returns True if all movements between shared parts of possibA and possibB are within limits, as specified by list partMovementLimits, which consists of (partNumber, maxSeparation) tuples.

• partNumber: Specified from 1 to n, where 1 is the soprano or highest part and n is the bass or lowest part.

• maxSeparation: For a given part, the maximum separation to allow between a pitch in possibA and a corresponding pitch in possibB, in semitones.

>>> from music21.figuredBass import possibility
>>> C4 = pitch.Pitch('C4')
>>> D4 = pitch.Pitch('D4')
>>> E4 = pitch.Pitch('E4')
>>> F4 = pitch.Pitch('F4')
>>> G4 = pitch.Pitch('G4')
>>> A4 = pitch.Pitch('A4')
>>> B4 = pitch.Pitch('B4')
>>> C5 = pitch.Pitch('C5')

Here, we limit the soprano part to motion of two semitones, enharmonically equivalent to a major second. Moving from C5 to B4 is allowed because it constitutes stepwise motion, but moving to A4 is not allowed because the distance between A4 and C5 is three semitones.

>>> partMovementLimits = [(1, 2)]
>>> possibA1 = (C5, G4, E4, C4)
>>> possibB1 = (B4, F4, D4, D4)
>>> possibility.partMovementsWithinLimits(possibA1, possibB1, partMovementLimits)
True
>>> possibB2 = (A4, F4, D4, D4)
>>> possibility.partMovementsWithinLimits(possibA1, possibB2, partMovementLimits)
False

music21.figuredBass.possibility.upperPartsSame(possibA, possibB)

Returns True if the upper parts are the same. False otherwise.

>>> from music21.figuredBass import possibility
>>> C4 = pitch.Pitch('C4')
>>> D4 = pitch.Pitch('D4')
>>> E4 = pitch.Pitch('E4')
>>> F4 = pitch.Pitch('F4')
>>> G4 = pitch.Pitch('G4')
>>> B4 = pitch.Pitch('B4')
>>> C5 = pitch.Pitch('C5')
>>> possibA1 = (C5, G4, E4, C4)
>>> possibB1 = (B4, F4, D4, D4)
>>> possibB2 = (C5, G4, E4, D4)
>>> possibility.upperPartsSame(possibA1, possibB1)
False
>>> possibility.upperPartsSame(possibA1, possibB2)
True

music21.figuredBass.possibility.couldBeItalianA6Resolution(possibA, possibB, threePartChordInfo=None, restrictDoublings=True)

Speed-enhanced but designed to stand alone. Returns True if possibA is an Italian A6 chord and possibB could possibly be an acceptable resolution. If restrictDoublings is set to True, only the tonic can be doubled. Setting restrictDoublings to False opens up the chance that the root or the third can be doubled. Controlled in the Rules object by restrictDoublingsInItalianA6Resolution.

>>> from music21.figuredBass import possibility
>>> A2 = pitch.Pitch('A2')
>>> Bb2 = pitch.Pitch('B-2')
>>> Cs4 = pitch.Pitch('C#4')
>>> D4 = pitch.Pitch('D4')
>>> E4 = pitch.Pitch('E4')
>>> Fs4 = pitch.Pitch('F#4')
>>> Gs4 = pitch.Pitch('G#4')
>>> A4 = pitch.Pitch('A4')
>>> possibA1 = (Gs4, D4, D4, Bb2)
>>> possibB1 = (A4, Cs4, E4, A2)
>>> possibB2 = (A4, E4, Cs4, A2)
>>> possibB3 = (A4, D4, Fs4, A2)
>>> possibility.couldBeItalianA6Resolution(possibA1, possibB1)
True
>>> possibility.couldBeItalianA6Resolution(possibA1, possibB1)
True
>>> possibility.couldBeItalianA6Resolution(possibA1, possibB3)
True

A PossibilityException is raised if possibA is not an Italian A6 chord, but this only applies if threePartChordInfo=None, because otherwise the chord information is coming from Segment and the fact that possibA is an It+6 chord is assumed.

>>> possibA2 = (Gs4, E4, D4, Bb2)
>>> possibB2 = (A4, E4, Cs4, A2)
>>> possibility.couldBeItalianA6Resolution(possibA2, possibB2)
Traceback (most recent call last):
music21.figuredBass.possibility.PossibilityException: possibA does not spell out an It+6 chord.

The method is called couldBeItalianA6Resolution as opposed to isItalianA6Resolution because it is designed to work in tandem with parallelOctaves() and isIncomplete() in a Segment. Consider the following examples with possibA1 above as the augmented sixth chord to resolve.

>>> possibA1 = (Gs4, D4, D4, Bb2)
>>> possibB4 = (A4, D4, D4, A2)  # No 3rd
>>> possibB5 = (A4, Cs4, Cs4, A2)  # No 5th
>>> possibility.couldBeItalianA6Resolution(possibA1, possibB4)
True
>>> possibility.couldBeItalianA6Resolution(possibA1, possibB5)  # parallel octaves
True

>>> possibA3 = (Gs4, Gs4, D4, Bb2)
>>> possibB6 = (A4, A4, Cs4, A2)
>>> possibility.couldBeItalianA6Resolution(possibA3, possibB6, restrictDoublings=True)
False
>>> possibility.couldBeItalianA6Resolution(possibA3, possibB6, restrictDoublings=False)
True

music21.figuredBass.possibility.limitPartToPitch(possibA, partPitchLimits=None)

Takes in a dict, partPitchLimits containing (partNumber, partPitch) pairs, each of which limits a part in possibA to a certain Pitch. Returns True if all limits are followed in possibA, False otherwise.

>>> from music21.figuredBass import possibility
>>> C4 = pitch.Pitch('C4')
>>> E4 = pitch.Pitch('E4')
>>> G4 = pitch.Pitch('G4')
>>> C5 = pitch.Pitch('C5')
>>> G5 = pitch.Pitch('G5')
>>> sopranoPitch = pitch.Pitch('G5')
>>> possibA1 = (C5, G4, E4, C4)
>>> possibility.limitPartToPitch(possibA1, {1: sopranoPitch})
False
>>> possibA2 = (G5, G4, E4, C4)
>>> possibility.limitPartToPitch(possibA2, {1: sopranoPitch})
True

music21.figuredBass.possibility.partsSame(possibA, possibB, partsToCheck=None)

Takes in partsToCheck, a list of part numbers. Checks if pitches at those part numbers of possibA and possibB are equal, determined by pitch space.

>>> from music21.figuredBass import possibility
>>> C4 = pitch.Pitch('C4')
>>> E4 = pitch.Pitch('E4')
>>> G4 = pitch.Pitch('G4')
>>> B4 = pitch.Pitch('B4')
>>> C5 = pitch.Pitch('C5')
>>> possibA1 = (C5, G4, E4, C4)
>>> possibB1 = (B4, G4, E4, C4)
>>> possibility.partsSame(possibA1, possibB1, [2, 3, 4])
True