The Nineteen Trichords

This post is a followup to Counting the 19 Trichords, where I looked at how we can list all the possible types of three-note chords that are distinct under transposition (shifting all notes up or down by the same interval) and inversion (displacing individual notes by an octave).  Here, I’d like to give a diagram for each of the nineteen trichords and say a little bit about its musical significance.

I’m using the numbering scheme from my previous post, where I organized the trichords in terms of their smallest interval, starting with trichords that contain two notes separated by a semitone, and progressing to the augmented triad (#19), where all notes are separated by major thirds.  See that post for a diagram explaining the numbering.  Here, I provide a clock diagram as the main illustration of each trichord, but you can also click on the View Star links in the comments to see an alternate “Star of David” diagram of each chord.

Trichord #1

Nickname: Semitone Cluster

Comments: [View Star]

Interval pattern: Reading clockwise from 12 o’clock, you’ll see that the first and second notes (in yellow) are separated by 1 semitone, the second and third are separated by 1 semitone, and the third and first are separated by 10 semitones.  So I list the interval pattern as [1 1 10] in semitones, or [m2 m2 m7] in interval names (i.e. m2 = minor second, etc.)

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Trichord #2

Nickname: Semi-Whole

Comments: Reading clockwise from 12 o’clock you could think of this as the first three notes of the Phyrgian mode.  Or, if you take the root at 1 o’clock, you have the outline of a Major 9th chord, including the Major 7th and 9th, but excluding the 3rd and 5th. [View Star]

Interval pattern: [1 2 9] or [m2 M2 M6]

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Trichord #3

Nickname: Minor/Major 7th

Comments: If you take the root at 1 o’clock, the other notes form a minor 3rd and major 7th above it, giving the signature of a “minor/major 7th chord,” missing a fifth. [View Star]

Interval pattern: [1 3 8] or [m2 m3 m6]

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Trichord #4

Nickname: Major 7th

Comments: If you take the root at 1 o’clock you have a major third and a major seventh above it, giving a major seventh chord without a fifth. [View Star]

Interval pattern: [1 4 7] or [m2 M3 P5]

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Trichord #5

Nickname: “Lydian Sus4”

Comments: I call this a “Lydian Sus4” because if you take the root at 6 o’clock, you have something that could be thought of as a Isus4 chord from the Lydian scale (i.e. where the 4 is sharp).   Alternatively, if you take the root at 12 o’clock, you have a b5 and b9 above it, so this could be seen as an incomplete 7b5b9 chord. [View Star]

Interval pattern: [1 5 6] or [m2 P4 T]

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Trichord #6

Nickname: “Phrygian Sus2”

Comments: With the root at 12 o’clock this could be thought of as a Isus2 from the Phrygian mode.  Or, if the root is at 1 o’clock, the chord could be a maj7b5 missing a third. [View Star]

Interval pattern: [1 6 5] or [m2 T P4]

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Trichord #7

Nickname: Major 7th Shell

Comments: With the root at 1 o’clock you have a Major seventh chord missing a third.  When the chord is missing a third I refer to it as a “shell” in the nickname, since we can’t tell what quality it has.  Compare with Trichord #4. [View Star]

Interval pattern: [1 7 4] or [m2 P5 M3]

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Trichord #8

Nickname: Augmented Major 7th

Comments: If the root is at 1 o’clock, this could be an Augmented Major 7th chord missing a third. [View Star]

Interval pattern: [1 8 3] or [m2 m6 m3]

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Trichord #9

Nickname: Whole-Semi

Comments: Starting at 10 o’clock, you can think of this as the first three notes of a minor scale, which can occur together in a minor ninth chord.  If we think of this as a minor ninth chord, it’s missing a fifth and seventh. [View Star]

Interval pattern: [1 9 2] or [m2 M6 M2]

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Trichord #10

Nickname: Whole Tone Cluster

Comments:  Starting at 12 o’clock, this can be seen as the first three notes of a major scale.  If we take the root at 2 o’clock, this could be seen as a minor or dominant ninth chord missing its third and fifth. [View Star]

Interval pattern: [2 2 8] or [M2 M2 m6]

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Trichord #11

Nickname: Minor 7th

Comments:  Take the root at 2 o’clock and you have a minor seventh chord without its fifth. [View Star]

Interval pattern: [2 3 7] or [M2 m3 P5]

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Trichord #12

Nickname: Dominant 7th

Comments:  Take the root at 2 o’clock and you have a major third and a minor seventh above it, creating a dominant seventh chord (missing its fifth). [View Star]

Interval pattern: [2 4 6] or [M2 M3 T]

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Trichord #13

Nickname: Suspended

Comments:  Depending on which note you treat as the root, this could be a Sus2 chord, a Sus4 chord, or a Quartal chord (stack of two perfect fourths). [View Star]

Interval pattern: [2 5 5] or [M2 P4 P4]

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Trichord #14

Nickname: Half-Diminished

Comments: If the root is at 2 o’clock we have a half-diminished seventh chord missing its third. If the root as at 8 o’clock we have majb5 chord. [View Star]

Interval pattern: [2 6 4] or [M2 T M3]

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Trichord #15

Nickname: Minor 7th Shell

Comments: If the root is at 2 o’clock this is a minor seventh chord missing its third.  Of course if we added a major third instead of a minor third, it would become a dominant seventh chord. [View Star]

Interval pattern: [2 7 3] or [M2 P5 m3]

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Trichord #16

Nickname: Diminished

Comments:  If the root is at 12 o’clock this is a diminished triad.  If the root is at 3 o’clock this could be thought of as a minor sixth chord (minor triad with added major sixth), without a fifth. [View Star]

Interval pattern: [3 3 6] or [m3 m3 T]

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Trichord #17

Nickname: Minor

Comments:  If the root is at 12 o’clock this is a minor triad.  With the root at 3 o’clock this could be thought of as a major sixth chord (major triad with added major sixth) missing a fifth. [View Star]

Interval pattern: [3 4 5] or [m3 M3 P4]

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Trichord #18

Nickname: Major

Comments:  With the root at 8 o’clock this is a major triad. [View Star]

Interval pattern: [3 5 4] or [m3 P4 M3]

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Trichord #19

Nickname: Augmented

Comments:  The only trichord with perfect symmetry!  Also, the only trichord that has fewer than 12 distinct instances as we transpose it (or rotate the template around the clock face). [View Star]

Interval pattern: [4 4 4] or [M3 M3 M3]

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