diff --git a/docs/_templates/layout.html b/docs/_templates/layout.html
new file mode 100644
index 0000000..1404cf7
--- /dev/null
+++ b/docs/_templates/layout.html
@@ -0,0 +1,18 @@
+{% extends "!layout.html" %}
+{% block footer %}
+{{ super() }}
+
+{% endblock %}
diff --git a/docs/guide/chords.rst b/docs/guide/chords.rst
index 90f52d5..c537ffb 100644
--- a/docs/guide/chords.rst
+++ b/docs/guide/chords.rst
@@ -27,6 +27,63 @@ harmony::
Minor 7th root + 3 + 7 + 10 Warm, mellow (Am7)
Diminished 7th root + 3 + 6 + 9 Dramatic, symmetrical
+Inversions
+----------
+
+A chord is in **root position** when the root is the lowest note.
+When a different chord tone is in the bass, the chord is **inverted**:
+
+- **Root position**: C E G (root in bass)
+- **First inversion**: E G C (3rd in bass) — notated C/E
+- **Second inversion**: G C E (5th in bass) — notated C/G
+
+Inversions change the color and weight of a chord without changing its
+identity. First inversion sounds lighter; second inversion sounds
+suspended, often used as a passing chord.
+
+For seventh chords, there's also **third inversion** (7th in bass):
+
+- G7 in third inversion: F G B D (notated G7/F)
+
+.. code-block:: python
+
+ from pytheory import Chord, Tone
+
+ # All three are "C major" — identify() finds the root
+ root = Chord([Tone.from_string(n, system="western") for n in ["C4", "E4", "G4"]])
+ first = Chord([Tone.from_string(n, system="western") for n in ["E3", "G3", "C4"]])
+ second = Chord([Tone.from_string(n, system="western") for n in ["G3", "C4", "E4"]])
+
+ root.identify() # 'C major'
+ first.identify() # 'C major'
+ second.identify() # 'C major'
+
+Extended Chords
+---------------
+
+Beyond seventh chords, jazz harmony builds **extended chords** by
+continuing to stack thirds:
+
+- **9th chord**: adds the 9th (= 2nd, one octave up)
+- **11th chord**: adds the 9th and 11th (= 4th)
+- **13th chord**: adds the 9th, 11th, and 13th (= 6th)
+
+A full 13th chord contains all 7 notes of the scale! In practice,
+tones are usually omitted — the 5th is typically dropped first, then
+the 11th (which clashes with the 3rd in dominant chords).
+
+.. code-block:: python
+
+ from pytheory import TonedScale
+
+ scale = TonedScale(tonic="C4")["major"]
+
+ # Build a Cmaj9 from the scale: C E G B D
+ cmaj9 = scale.chord(0, 2, 4, 6, 8)
+
+ # Build a full C13 (in theory): C E G B D F A
+ c13 = scale.chord(0, 2, 4, 6, 8, 10, 12)
+
Using the Chord Chart
---------------------
diff --git a/docs/guide/scales.rst b/docs/guide/scales.rst
index 8069f9f..a9a070c 100644
--- a/docs/guide/scales.rst
+++ b/docs/guide/scales.rst
@@ -208,6 +208,60 @@ Some of the most-used chord progressions in Western music:
- **I–IV–V–I** — the foundation of blues, rock, country, folk
- **I–V–vi–IV** — the "pop progression" (Let It Be, No Woman No Cry,
- With or Without You)
+ With or Without You, Someone Like You)
- **ii–V–I** — the backbone of jazz harmony
- **I–vi–IV–V** — the "50s progression" (Stand By Me, Every Breath You Take)
+- **i–bVI–bIII–bVII** — the "epic" minor progression (Stairway to Heaven,
+ My Heart Will Go On)
+- **I–IV–vi–V** — axis of awesome (many, many pop songs)
+
+The 12-Bar Blues
+~~~~~~~~~~~~~~~~
+
+The **12-bar blues** is the most influential chord progression in
+American music. It's 12 measures long and uses only three chords
+(I, IV, V)::
+
+ | I | I | I | I |
+ | IV | IV | I | I |
+ | V | IV | I | V |
+
+Every blues, early rock and roll, and much of jazz is built on this
+structure. In the key of A::
+
+ | A | A | A | A |
+ | D | D | A | A |
+ | E | D | A | E |
+
+.. code-block:: python
+
+ from pytheory import TonedScale
+
+ a = TonedScale(tonic="A4")["major"]
+ I = a.triad(0) # A major
+ IV = a.triad(3) # D major
+ V = a.triad(4) # E major
+
+ # The 12-bar blues progression
+ blues_12 = [I, I, I, I, IV, IV, I, I, V, IV, I, V]
+
+Parallel Major and Minor
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Two scales are **relative** if they share the same notes (C major and
+A minor). Two scales are **parallel** if they share the same tonic but
+have different notes (C major and C minor).
+
+Mixing parallel major and minor is a powerful compositional tool —
+borrowing chords from the parallel minor in a major key creates
+dramatic color shifts. The bVI and bVII chords (Ab and Bb in C major)
+are borrowed from C minor and appear constantly in rock and film music.
+
+.. code-block:: python
+
+ c_major = TonedScale(tonic="C4")["major"]
+ c_minor = TonedScale(tonic="C4")["minor"]
+
+ # Compare: same tonic, different notes
+ c_major.note_names # ['C', 'D', 'E', 'F', 'G', 'A', 'B', 'C']
+ c_minor.note_names # ['C', 'D', 'D#', 'F', 'G', 'G#', 'A#', 'C']
diff --git a/docs/guide/theory.rst b/docs/guide/theory.rst
new file mode 100644
index 0000000..8102c64
--- /dev/null
+++ b/docs/guide/theory.rst
@@ -0,0 +1,254 @@
+Music Theory Fundamentals
+=========================
+
+This page covers the essential concepts of music theory — the framework
+behind everything PyTheory does.
+
+Sound and Pitch
+---------------
+
+All sound is vibration. When an object vibrates, it pushes air molecules
+back and forth, creating pressure waves that travel to your ears. The
+speed of this vibration — measured in cycles per second (Hertz, Hz) —
+determines the **pitch** you hear.
+
+- **20 Hz**: the lowest pitch most humans can hear
+- **60–250 Hz**: the range of the human voice (speaking)
+- **261.63 Hz**: middle C (C4)
+- **440 Hz**: the tuning standard A (A4)
+- **4186 Hz**: the highest C on a piano (C8)
+- **20,000 Hz**: the upper limit of human hearing
+
+The relationship between pitch and frequency is **logarithmic** — each
+octave doubles the frequency. This means the distance from A3 (220 Hz)
+to A4 (440 Hz) is 220 Hz, but the distance from A4 to A5 (880 Hz) is
+440 Hz. Both sound like "one octave" to our ears.
+
+Why Twelve Notes?
+-----------------
+
+The Western chromatic scale has 12 notes per octave. This isn't arbitrary —
+it emerges from the physics of vibrating strings and air columns.
+
+The **harmonic series** is the sequence of frequencies produced when a
+string vibrates: f, 2f, 3f, 4f, 5f... The relationships between these
+harmonics create the intervals we perceive as consonant:
+
+- 2:1 = octave (the most fundamental)
+- 3:2 = perfect fifth
+- 4:3 = perfect fourth
+- 5:4 = major third
+- 6:5 = minor third
+
+If you stack perfect fifths (multiply by 3/2 repeatedly) and reduce to
+within one octave, you get 12 roughly evenly-spaced notes before the
+cycle almost closes. The tiny gap where it doesn't close perfectly is
+the **Pythagorean comma** — the reason we need temperament.
+
+.. code-block:: python
+
+ from pytheory import Tone
+
+ # Walk the circle of fifths — all 12 notes
+ c = Tone.from_string("C4", system="western")
+ [t.name for t in c.circle_of_fifths()]
+ # ['C', 'G', 'D', 'A', 'E', 'B', 'F#', 'C#', 'G#', 'D#', 'A#', 'F']
+
+Other cultures divide the octave differently: Indonesian gamelan uses
+5 or 7 unequal divisions; Indian classical music theoretically has 22
+shrutis (microtones); Arabic maqam uses quarter-tones.
+
+Intervals: The Atoms of Music
+------------------------------
+
+An **interval** is the distance between two pitches. Intervals are the
+building blocks of everything — melodies are sequences of intervals,
+chords are stacks of intervals, and scales are patterns of intervals.
+
+Every interval has two properties:
+
+**Size** (how many scale steps)::
+
+ Unison → 2nd → 3rd → 4th → 5th → 6th → 7th → Octave
+
+**Quality** (exact number of semitones)::
+
+ Perfect: unison (0), 4th (5), 5th (7), octave (12)
+ Major: 2nd (2), 3rd (4), 6th (9), 7th (11)
+ Minor: 2nd (1), 3rd (3), 6th (8), 7th (10)
+ Augmented: one semitone larger than perfect or major
+ Diminished: one semitone smaller than perfect or minor
+
+The "perfect" intervals (unison, 4th, 5th, octave) are called perfect
+because they appear in both major AND minor scales unchanged. They've
+been considered consonant across virtually all musical cultures
+throughout history.
+
+The **tritone** (augmented 4th / diminished 5th = 6 semitones) divides
+the octave exactly in half. Medieval theorists called it *diabolus in
+musica* ("the devil in music") because of its extreme instability.
+Today it's the foundation of dominant harmony and the blues.
+
+Keys and Key Signatures
+-----------------------
+
+A **key** is a group of notes that form the tonal center of a piece.
+The key of C major uses only the white keys on the piano: C D E F G A B.
+The key of G major uses the same notes except F becomes F#.
+
+Key signatures tell you which notes are sharped or flatted throughout
+a piece. They follow the circle of fifths:
+
+**Sharp keys** (add one sharp per step clockwise)::
+
+ C major: no sharps or flats
+ G major: F#
+ D major: F# C#
+ A major: F# C# G#
+ E major: F# C# G# D#
+ B major: F# C# G# D# A#
+
+**Flat keys** (add one flat per step counter-clockwise)::
+
+ C major: no sharps or flats
+ F major: Bb
+ Bb major: Bb Eb
+ Eb major: Bb Eb Ab
+ Ab major: Bb Eb Ab Db
+ Db major: Bb Eb Ab Db Gb
+
+The order of sharps is always F C G D A E B (Father Charles Goes Down
+And Ends Battle). The order of flats is the reverse: B E A D G C F.
+
+Harmony: How Chords Work
+-------------------------
+
+**Harmony** is the art of combining tones simultaneously. While melody
+is horizontal (tones in sequence), harmony is vertical (tones stacked).
+
+The simplest harmony is the **triad** — three notes built by stacking
+thirds. The quality of each third determines the chord type:
+
+- **Major triad** = major 3rd + minor 3rd (e.g. C-E-G)
+- **Minor triad** = minor 3rd + major 3rd (e.g. C-Eb-G)
+- **Diminished triad** = minor 3rd + minor 3rd (e.g. B-D-F)
+- **Augmented triad** = major 3rd + major 3rd (e.g. C-E-G#)
+
+In any major key, the triads built on each scale degree always follow
+the same pattern::
+
+ Degree Quality Function
+ I Major Tonic (home)
+ ii Minor Pre-dominant
+ iii Minor Tonic substitute
+ IV Major Subdominant (departure)
+ V Major Dominant (tension, wants to go home)
+ vi Minor Tonic substitute, relative minor
+ vii° Diminished Dominant substitute (leading tone chord)
+
+This pattern is the DNA of Western harmony. Pop songs, classical
+sonatas, jazz standards, and church hymns all derive from it.
+
+Functional Harmony
+~~~~~~~~~~~~~~~~~~
+
+Chords don't just have names — they have **functions**:
+
+- **Tonic function** (I, iii, vi): stability, rest, home
+- **Subdominant function** (ii, IV): motion away from home
+- **Dominant function** (V, vii°): tension, desire to return home
+
+The most fundamental progression in Western music is **T → S → D → T**
+(tonic → subdominant → dominant → tonic). The classic I-IV-V-I is
+exactly this pattern. Every "Louie Louie" and every Bach chorale follows
+this basic tonal gravity.
+
+.. code-block:: python
+
+ from pytheory import TonedScale
+
+ scale = TonedScale(tonic="C4")["major"]
+
+ # The I-IV-V-I progression
+ I = scale.triad(0) # C major — home
+ IV = scale.triad(3) # F major — departure
+ V = scale.triad(4) # G major — tension
+ # I again # C major — resolution
+
+The Dominant Seventh
+~~~~~~~~~~~~~~~~~~~~
+
+The most important chord in tonal music is the **dominant seventh** —
+the V7 chord. In C major, this is G-B-D-F. It contains:
+
+- A **leading tone** (B) that pulls up to the tonic (C) by half step
+- A **tritone** (B-F) that wants to resolve inward (B→C, F→E)
+- The **dominant note** (G) that falls to the tonic by a fifth
+
+This combination creates the strongest possible pull toward resolution.
+When you hear V7→I, you feel arrival.
+
+.. code-block:: python
+
+ from pytheory import Chord, Tone
+
+ C4 = Tone.from_string("C4", system="western")
+ G4 = Tone.from_string("G4", system="western")
+
+ g7 = Chord([G4, G4+4, G4+7, G4+10]) # G B D F
+ g7.identify() # 'G dominant 7th'
+ g7.tension['has_dominant_function'] # True
+ g7.tension['tritones'] # 1
+
+ c_major = Chord([C4, C4+4, C4+7]) # C E G
+ c_major.tension['score'] # 0.0 — fully resolved
+
+Rhythm and Meter
+----------------
+
+While PyTheory focuses on pitch, rhythm is the other half of music.
+
+**Rhythm** is the pattern of durations. **Meter** is the recurring
+pattern of strong and weak beats that organizes rhythm.
+
+- **4/4 time**: the most common meter. Strong-weak-medium-weak.
+ Used in rock, pop, hip-hop, most Western music.
+- **3/4 time**: waltz time. Strong-weak-weak. A lilting, circular feel.
+- **6/8 time**: compound duple. Two groups of three. Irish jigs, many
+ ballads.
+- **5/4 time**: asymmetric. "Take Five" by Dave Brubeck. Creates
+ constant forward momentum because it never fully settles.
+- **7/8 time**: common in Balkan folk music. Often felt as 2+2+3 or
+ 3+2+2.
+
+The Physics of Consonance
+-------------------------
+
+Why do some intervals sound "good" and others "bad"? The answer lies
+in the physics of sound waves.
+
+When two frequencies are related by a simple ratio (like 3:2 for a
+perfect fifth), their waveforms align regularly. The combined wave
+is smooth and periodic — the brain perceives this as consonant.
+
+When two frequencies are related by a complex ratio (like 45:32 for
+a tritone), their waveforms rarely align. The combined wave is
+irregular and the brain perceives roughness — dissonance.
+
+But consonance and dissonance are also cultural. The major third (5:4)
+was considered dissonant in medieval European music but consonant since
+the Renaissance. The tritone was forbidden in church music but is the
+foundation of blues and jazz. Indonesian gamelan embraces beating
+between paired instruments as a core aesthetic.
+
+.. code-block:: python
+
+ from pytheory import Chord, Tone
+
+ C4 = Tone.from_string("C4", system="western")
+
+ # The overtone series explains why fifths sound consonant
+ C4.overtones(6)
+ # [261.63, 523.25, 784.88, 1046.50, 1308.13, 1569.75]
+ # The 3rd harmonic (784.88) is very close to G5 (783.99)
+ # — the fifth is "built into" the tone itself
diff --git a/docs/guide/tones.rst b/docs/guide/tones.rst
index 587613d..d27a9cd 100644
--- a/docs/guide/tones.rst
+++ b/docs/guide/tones.rst
@@ -199,6 +199,62 @@ Equality checks note name and octave:
>>> c4 == Tone(name="C", octave=4)
True
+The Overtone Series
+-------------------
+
+Every tone you hear is actually a composite of many frequencies. When
+a string vibrates, it doesn't just vibrate as a whole — it also vibrates
+in halves, thirds, quarters, and so on, producing the **harmonic series**:
+
+.. code-block:: python
+
+ >>> a4 = Tone.from_string("A4", system="western")
+ >>> a4.overtones(8)
+ [440.0, 880.0, 1320.0, 1760.0, 2200.0, 2640.0, 3080.0, 3520.0]
+
+These harmonics correspond to musical intervals::
+
+ Harmonic Frequency Interval from fundamental
+ 1st 440 Hz Unison (A4)
+ 2nd 880 Hz Octave (A5)
+ 3rd 1320 Hz Octave + perfect 5th (E6)
+ 4th 1760 Hz Two octaves (A6)
+ 5th 2200 Hz Two octaves + major 3rd (C#7)
+ 6th 2640 Hz Two octaves + perfect 5th (E7)
+ 7th 3080 Hz Two octaves + minor 7th (≈G7, slightly flat)
+ 8th 3520 Hz Three octaves (A7)
+
+The overtone series is why a perfect fifth sounds consonant — the 3rd
+harmonic of the lower note matches the 2nd harmonic of the upper note.
+It's also why the major triad (root, major 3rd, perfect 5th) feels
+"natural" — these intervals appear in the first 6 harmonics.
+
+Different instruments emphasize different harmonics, which is why a
+violin and a flute playing the same note sound different. This quality
+is called **timbre**.
+
+Enharmonic Equivalents
+----------------------
+
+In equal temperament, C# and Db are the same pitch (they have the
+same frequency). They're called **enharmonic equivalents**. Which name
+you use depends on context:
+
+- In the key of **D major** (2 sharps), you write **C#**
+- In the key of **Gb major** (6 flats), you write **Db**
+
+The rule: each letter name should appear exactly once in a scale. The
+D major scale is D E F# G A B C# — not D E Gb G A B Db, even though
+F#=Gb and C#=Db.
+
+PyTheory uses sharps by default (following the tone list ordering), but
+tones carry their enharmonic equivalents:
+
+.. code-block:: python
+
+ >>> Tone.from_tuple(("C#", "Db")).names()
+ ['C#', 'Db']
+
The Circle of Fifths
--------------------
diff --git a/docs/index.rst b/docs/index.rst
index 26abb52..9225ddb 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -27,6 +27,7 @@ Work with tones, scales, chords, and fretboards using a clean, Pythonic API.
:caption: User Guide
guide/quickstart
+ guide/theory
guide/tones
guide/scales
guide/chords