Module possibility 

The rules a possibility is held to: music21’s figuredBass.possibility.

A possibility is one way of voicing a figured-bass chord: a pitch for each part, from the highest part down to the bass, which is last. Unlike a chord’s notes, the order says which part sings which pitch, and parts may cross, so the highest pitch need not be first.

The rules come in two kinds. Those over one possibility say whether a voicing is acceptable on its own; those over two say whether moving from one voicing to the next is.

Structs

ItalianSixth

The notes of an Italian augmented sixth an Italian-sixth resolution is judged by: its bass, root, third and fifth.

Statics

DEFAULT_MAX_PITCH

The highest pitch a part may reach unless told otherwise: B5.

Functions

could_be_italian_a6_resolution

Whether moving from a, an Italian augmented sixth, to b resolves it: the bass falls a semitone, the root rises one, the third falls one and the fifth moves by a third or a second. With restrict_doublings, the root may resolve only once and the third not at all.

hidden_fifths

Whether the outer parts, the highest and the bass, move by similar motion into a fifth from a to b.

hidden_octaves

Whether the outer parts, the highest and the bass, move by similar motion into an octave from a to b.

is_incomplete

Whether the possibility leaves out any of pitch_names, the names of the notes its chord has to sound.

limit_part_to_pitch

Whether each part named in part_pitch_limits sings exactly the pitch it is held to. Parts are numbered from one, the highest.

parallel_fifths

Whether any two parts move in parallel fifths from a to b.

parallel_octaves

Whether any two parts move in parallel octaves from a to b.

part_movements_within_limits

Whether each part named in limits moves no further than the semitones beside it from a to b. Parts are numbered from one, the highest.

part_pairs

Each part’s pitch in a beside its pitch in b.

parts_same

Whether each part named in parts sings the same pitch in a and b; with no parts named, they always do. Parts are numbered from one, the highest.

pitches_within_limit

Whether no part sings above max_pitch, which is DEFAULT_MAX_PITCH unless a caller says otherwise.

upper_parts_same

Whether every part but the bass sings the same pitch in a and b.

upper_parts_within_limit

Whether every two of the upper parts – all but the bass – lie within max_semitone_separation of each other; with no limit, they always do. music21’s default is twelve, the span a hand can play.

voice_crossing

Whether any part sings higher than a part above it.

voice_overlap

Whether moving from a to b takes any part past where a part above it was, or below where a part beneath it was.