chord-lab-with-pytheory

# Chord Lab Everything about a *single* chord — construction, voicing, and analysis. ## Build a chord ```python 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`: ```python 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 ```python 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 ```python 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 ```python 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. ```python 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): ```python 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) ```python 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. ```python 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. - `tension` returns a dict; `tension["score"]` is the scalar. - To *identify* a chord from notes or fret positions, that's the guitar skill (`Fingering.identify()`) or `Chord(...).identify()`.