kennethreitz Chord Lab
Everything about a single chord — construction, voicing, and analysis.
Build a chord
from pytheory import Chord
Chord.from_symbol("F#m7b5") # widest parser: sus, add9, alterations, slash
Chord.from_name("Am") # plain names (major/minor/7/maj7/dim/…)
Chord.from_intervals("C", 0, 4, 7) # root + semitone intervals
Chord.from_tones("C", "E", "G") # explicit notes
A Chord has .symbol, .tones (list of Tone), .root, and
.transpose(semitones).
Voicings
inversion, drop2, drop3, and open_voicing return a new Chord. The
chord symbol stays the same — the change is in the tones (order/octave), so
inspect .tones:
c = Chord.from_symbol("Cmaj7")
[str(t) for t in c.tones] # ['C4', 'E4', 'G4', 'B4']
[str(t) for t in c.inversion(1).tones] # ['E4', 'G4', 'B4', 'C5']
[str(t) for t in c.drop2().tones] # ['G3', 'C4', 'E4', 'B4']
[str(t) for t in c.open_voicing().tones] # ['C4', 'E5', 'G4', 'B5']
drop3() exists too. Use these to spread a close voicing for piano/strings/guitar.
Analysis
c = Chord.from_symbol("Cmaj7")
c.intervals # [4, 3, 4] semitone steps between stacked tones
c.pitch_classes # {0, 4, 7, 11}
c.forte_number # '4-20' set-theory label
c.figured_bass # '7'
c.extensions() # [<Tone D5>, <Tone A5>] available 9/11/13 tones
c.tension # {'score': 0.15, 'tritones': 0, 'minor_seconds': 1,
# 'has_dominant_function': False}
c.dissonance # 5.33 (a roughness number; higher = more dissonant)
c.beat_frequencies # [(Tone, Tone, hz), …] beating between pairs in ET
Pitch-class-set toolkit
c = Chord.from_symbol("Cmaj7")
c.normal_form # (11, 0, 4, 7) most compact ordering
c.prime_form # (0, 1, 5, 8) canonical set-class form
c.interval_vector # (1, 0, 1, 2, 2, 0) interval-class content <ic1..ic6>
c.complement # Chord of the other 8 pitch classes
# Set-class relationships between two chords:
Chord.from_symbol("C").is_transposition_of(Chord.from_symbol("G")) # True (Tn)
Chord.from_symbol("C").is_set_class_equivalent(Chord.from_symbol("Cm")) # True (TnI: maj/min)
Chord.from_symbol("C").is_subset_of(Chord.from_symbol("Cmaj7")) # True
# Z-relation — same interval vector, different set class (e.g. 4-z15 / 4-z29):
a, b = Chord.from_midi_message(0,1,4,6), Chord.from_midi_message(0,1,3,7)
a.is_z_related(b) # True
Reharmonization & voice leading
Chord.from_symbol("G7").tritone_sub() # -> Db7 (the classic sub)
# Negative harmony — mirror across a key's tonic↔dominant axis (Levy/Collier):
Chord.from_symbol("C").negative_harmony("C").identify() # 'C minor'
Chord.from_symbol("G7").negative_harmony("C") # the negative dominant
# (Key("C","major").negative_harmony() gives the axis, hinge notes, and bridge
# chord — that's the keys-and-harmony skill.)
# Reharmonization ideas for a chord in a key (tritone sub, diatonic subs,
# secondary dominant, negative harmony) — one dict per suggestion:
from pytheory import reharmonize
for s in reharmonize(Chord.from_symbol("G7"), "C"):
print(s["technique"], "->", s["chord"].identify())
# Or from the shell: pytheory reharmonize G7 --key C (--json / --play too)
# Reharmonize a whole progression (techniques: secondary_dominants/tritone/diatonic):
from pytheory import reharmonize_progression
prog = [Chord.from_symbol(s) for s in ("C","Am","Dm","G7","C")]
[c.symbol for c in reharmonize_progression(prog, "C", technique="secondary_dominants")]
# ['C','E7','Am','A7','Dm','D7','G7','C'] — the cycle-of-dominants reharm
# Smoothest motion from one chord to the next, voice by voice:
for frm, to, semis in Chord.from_symbol("Cmaj7").voice_leading(Chord.from_symbol("Fmaj7")):
print(frm, "->", to, f"({semis:+d} semitones)")
Neo-Riemannian (P/L/R) — chromatic triad moves
The P/L/R transformations move a single voice to flip a major/minor triad into another — the engine behind Tonnetz harmony and a lot of film-score chromaticism. Each is its own inverse; together they reach all 24 triads.
C = Chord.from_symbol("C")
C.parallel().identify() # 'C minor' (P: same root, flip quality)
C.relative().identify() # 'A minor' (R: relative)
C.leading_tone_exchange().identify() # 'E minor' (L: Leittonwechsel)
C.transform("LP").identify() # 'E major' (apply a sequence)
C.tonnetz_path(Chord.from_symbol("Am")) # 'R' — shortest P/L/R route between triads
C.tonnetz_path(Chord.from_symbol("Abm")) # 'PLP' — the hexatonic pole of C major
Part-writing checker (parallels / crossing)
check_voice_leading flags the common-practice no-no's across a sequence of
voicings. Each voicing's tones are read low-to-high as the voices (so a
4-note chord gets bass/tenor/alto/soprano labels):
from pytheory import Chord, check_voice_leading
a = Chord.from_midi_message(48, 55) # C3 + G3 (a fifth)
b = Chord.from_midi_message(50, 57) # D3 + A3 (a fifth) — both rise
check_voice_leading([a, b])
# [{'type': 'parallel fifths', 'chords': (0, 1), 'voices': (0, 1),
# 'description': 'parallel fifths between voice 1 and voice 2 (chords 0→1)'}]
It catches parallel fifths, parallel octaves, and voice
crossing; clean part-writing returns [].
Chord-scale theory (what to solo with)
from pytheory import Chord, chord_scales, chord_scale_notes, avoid_notes
chord_scales(Chord.from_symbol("G7")) # ['mixolydian']
chord_scales(Chord.from_symbol("Cm7")) # ['dorian','aeolian','phrygian']
chord_scales(Chord.from_symbol("Em7"), key="C") # ['phrygian', …] diatonic mode first
[t.name for t in chord_scale_notes(Chord.from_symbol("Cmaj7"))] # C D E F G A B
[t.name for t in avoid_notes(Chord.from_symbol("Cmaj7"))] # ['F'] (½-step above the 3rd)
chord_scales ranks scales by fit (quality alone, or the diatonic mode
first when you pass a key); avoid_notes flags scale tones a half-step
above a chord tone.
from pytheory.play import play, save
from pytheory import Chord, Synth
play(Chord.from_symbol("Cmaj7"), t=2000) # through the speakers
save(Chord.from_symbol("Cmaj7"), "chord.wav", t=2000, synth=Synth.TRIANGLE)
Tips
- Voicing methods change
.tones, not.symbol— compare the tone lists. tensionreturns a dict;tension["score"]is the scalar.- To identify a chord from notes or fret positions, that's the guitar skill
(
Fingering.identify()) orChord(...).identify().