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Home›Music theory

Chords

Stack thirds to build triads, hear the difference between major, minor, diminished and augmented, name every chord in a key with Roman numerals, read inversions and seventh chords, and follow the progressions and cadences that end a musical phrase.

Progress

About this course

Doubling and spacing do not change a chord's name. A guitarist strumming six strings on an open C chord plays C, E, G, C, E, with one string silent, and a pianist might play C in the left hand and E, G, C across the right. Both are still a C major triad, because the set of letter names is the same. This course builds chords from that idea: stack thirds to make triads, then name and hear what you have built.

The six units are triads, major, minor, diminished and augmented, chords in a key and Roman numerals, inversions and figured bass, seventh chords, and progressions and cadences. Two minor thirds make a diminished triad: B, D, F is 3 + 3 half steps, with a diminished fifth of only 6. Two major thirds make an augmented triad: C, E, G♯ is 4 + 4, and its fifth is 8 half steps. In a major key the diminished triad appears once, on the leading tone, and the augmented triad does not appear at all among the seven triads built from the scale. Build a seventh chord on each degree of a major scale and the qualities fall into a fixed pattern. In C major, G7 is the only dominant seventh, Dm7, Em7 and Am7 are minor sevenths, and Bø7 is half-diminished.

The last unit gives phrases a shape. In this style phrases usually move from tonic to predominant to dominant and back to tonic, written T-PD-D-T, which textbooks call the phrase model. I-IV-V-I in C major is C, F, G, C, a phrase that leaves home, builds tension and returns. Add F on top of G-B-D and you get G7, a chord whose B wants to rise to C while its F wants to fall to E.

Before you start

Thirds and major scales help explain nearly every chord on these pages. Reviewing Intervals and Scales and keys helps you follow the note choices rather than memorise shapes. The lessons start by stacking thirds, then keep the same notes in view as chords change position and function.

When you finish

You can build major, minor, diminished and augmented triads, label chords within a key with Roman numerals, and recognise inversions from the bass note. You can add a seventh to a triad, follow a chord progression, and distinguish a phrase ending on V from one that moves from V to I.

  1. 1TriadsSkip every other white key from C (C, E, G) and you have played a triad, the three-note chord under most of the songs you know.
  2. 2Major, minor, diminished and augmentedC-E-G and C-E♭-G differ by one half step in the middle note, and that single half step is the difference between a major and a minor chord.
  3. 3Chords in a key and Roman numeralsBuild a triad on each of the seven notes of any major scale and the pattern is always the same: three major, three minor and one diminished.
  4. 4Inversions and figured bassMove the C of a C major chord to the top and the chord is still C major, but with E in the bass it sounds lighter and less settled.
  5. 5Seventh chordsAdd F on top of G-B-D and you get G7, a chord whose B wants to rise to C while its F wants to fall to E.
  6. 6Progressions and cadencesEnd a phrase on V and it sounds like a question; follow V with I and it sounds like a full stop.

Related courses

  • Reading notes
    Learn to read pitches on the staff: treble clef, bass clef, ledger lines, the grand staff, sharps and flats, and the different names musicians use for the same notes. Every unit has a staff quiz or a keyboard you can play.
  • Rhythm and meter
    How music is measured in time: the beat and tempo, note values and rests, dots and ties, simple meters like 3/4 and 4/4, compound meters like 6/8, and syncopation, triplets and pickups. Every unit has the rhythm lab with a tap-along test that scores each tap in milliseconds, and most units add a metronome for any time signature and note blocks you stack into bars.
  • Scales and keys
    Build any major scale from one pattern of whole and half steps, name every scale degree, read a key signature at a glance, find your way around the circle of fifths, and tell a minor key from its relative major.
  • Intervals
    An interval is the distance between two notes. Count half steps on the keyboard, name thirds, fourths, fifths and octaves, hear why some intervals sound stable, and train your ear and voice with melodies everyone knows.
Sources for this course (9)
  • Open Music Theory 2e (Viva Pressbooks; ideas only, no sentences copied): "Triads" (a chord is three or more pitch classes sounding together, triads stack in thirds on all lines or all spaces, root/third/fifth, four qualities named for the third (major, minor) or the fifth (diminished, augmented), common expressive associations, chord symbols C, Cmi, C°, C+, spelling with the root's major key signature, doubling and spacing do not change identity, triad qualities on each degree of major and minor; viva.pressbooks.pub/openmusictheory/chapter/triads/)
  • "Seventh Chords" (five qualities, other names, qualities on each degree in major; viva.pressbooks.pub/openmusictheory/chapter/seventh-chords/)
  • "Inversion" (bass vs root, first/second/third inversion, slash symbols; viva.pressbooks.pub/openmusictheory/chapter/inversion/)
  • "Figured Bass and Roman Numerals with Figures" (5/3, 6/3, 6/4, 7, 6/5, 4/3, 4/2, abbreviations; viva.pressbooks.pub/openmusictheory/chapter/figured-bass/)
  • "Roman Numerals" (uppercase major, lowercase minor, ° diminished, + augmented, ø7 and °7; viva.pressbooks.pub/openmusictheorycopy/chapter/roman-numerals)
  • "Introduction to Harmony, Cadences, and Phrase Endings" (tonic, predominant, dominant functions with chord lists, phrase model, PAC/IAC/HC definitions; viva.pressbooks.pub/openmusictheory/chapter/intro-to-harmony/)
  • Robert Hutchinson, Music Theory for the 21st-Century Classroom, "Cadences" (plagal IV-I, deceptive V-vi, the prelude to Act I of Wagner's Tristan und Isolde as a famous deceptive cadence; no bar number given in the text; musictheory.pugetsound.edu/mt21c/cadences.html) and "Figured Bass Inversion Symbols" (musictheory.pugetsound.edu/mt21c/FiguredBassInversionSymbols.html)
  • Wikipedia "Treatise on Harmony" (Rameau, Paris 1722, fundamental bass and inversion of chords; en.wikipedia.org/wiki/Treatise_on_Harmony)
  • musictheory.net lessons 40-47 used as a cross-check only. frequencies with f = 440 x 2^((m - 69) / 12) (C4 261.63, E4 329.63, G4 392.00 Hz).

Checked: 2026-10-17