Chord Structure for the Major Scale (Ionian Mode)

Building on the Intervals and Scales post, which can be viewed here. Let’s examine chord structure for the Major scale, specifically triads. Below are the scale degrees and their names for C Ionian.  

Notes can be played melodically (horizontal) or harmonically (vertical). 3 or more notes played harmonically together are called a chord. There are a plethora of chords, from triads to thirteenths. Extensions are when you add a 9th, 11th, or 13th above the seventh of the chord. The main type of chords are Major, Minor, Diminshed, Half-Diminshed, Augmented, Dominant. You can even have secundal (seconds) quartal (fourths),  and quintal (fifths) harmonies.

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You can build tertian triads by starting on each scale degree and going diatonically (notes in the scale) add a third on top of each note. So if we started on C and went diatonically up a third we would come to E, then starting on E and went diatonically up a third we would come to G. If we combine those together we get the chord of C.

Diatonic chords are made up of major and minor thirds. A major third is four half-steps and a minor third is three half steps. There are 4 types of triads: Major, Minor, Diminshed, and Augmented.

To build the 7 tertian triads in the diatonic scale you use: Major, Minor, and Diminished.

Major triads are mathematically constructed as a Major 3rd with a Minor 3rd on top. So looking at the C chord we know it’s a major chord because C to E is a Major 3rd and E to G is a minor 3rd. (MA3 + Mi3)

Minor triads are mathematically constructed as a Minor 3rd with a Major 3rd on top. So, going to the sixth degree of the C major scale, A, we see that it is a minor chord because A to C is a minor 3rd and C to E is a Major 3rd. (Mi3 + MA3)

Diminished triads are mathematically constructed as a Minor 3rd with a Minor 3rd on top so two minor 3rds. So, going to the seventh degree of the C major scale, we see that B is diminished because B to D is a minor 3rd and D to F is a minor 3rd. (Mi3 + Mi3)

 Note the construction of an Augmented triad is a major third with a major third on top: MA3 + MA3. An example would be C augmented where there is a major third from C to E and a Major third from E to G#.

The lowest note when stacked in thirds is the root (1), the middle note is a third (3), and the highest note is the Fifth (5). So, in the G Major Triad, the Root is G (1), the third is B (3), and the Fifth is D (5). All intervals are based on the root of the chord. So the 5th of the G Chord is D because it is a fifth diatonically from G.

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To write chords we use Roman numerals to associate the chords in each key. For chords with a major third as its first interval, we use upper-case Roman numerals. For chords with a minor third as its first interval, we use lowercase Roman numerals. We assign the chord name to the scale degree it starts on. So, in the key of C Major, G Major is written as a V since it is on the 5th degree of the scale and has a major third as its first interval. To denote a diminished triad, like B diminished, we use the degree symbol ° after the Roman numeral. Here are all the diatonic triads in the C Ionian Scale.

Learning to use the Roman numeral system is advantageous as it allows you to easily write out a schematic to a song in short-hand harmonic notation. This makes transposing a song and playing it in a different key really easy and convenient. Here is a sample of a schematic. Though it says the key of D, it could easily be played in any Major key. Chords can be played in inversions where another note other than the root is the lowest note. For details on chord inversions and marking them see the post here.

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We divide the chords in the scale into two classifications: Primary and Secondary triads. Primary triads are the chords on the 1st (tonic), 4th (subdominant), and 5th (dominant) degrees of the scale. Secondary triads are the chords on the 2nd (supertonic), 3rd (mediant), 6th (submediant), and 7th (leading tone) degrees of the scale. You will notice that the Primary triads are related by Perfect 5ths. C up to G and C down to F. So, using Roman numerals the Primary and Secondary triads are as follows:

Primary triads: I, IV, and V

Secondary triads: ii, iii, vi, and  vii°

The chords for the Ionian mode are all structured the same. If you start on a pitch and create the Ionian mode (constructed in seconds is MA2, MA2, Mi2, MA2, MA2, MA2, Mi2), the I, IV, and V chords will be major, the ii, iii, and vi chords will be minor, and the vii° chord will be diminished.