Why do we only build 7th chords on the supertonic and dominant of a major scale?

Question

I am just cracking into a book on basic Harmony (Mark Sarnecki, 2nd edition), and the first section begins with a review of the triads built on each degree of a C major scale, as shown in this picture.

Then after introducing the various types of 7th chords, the same sequence of chords is considered, along 7th chords built on the supertonic and dominant of the C major scale, as shown below.

It results in a minor 7th chord built on D, and a dominant seventh chord build on G.

My actual question:

It seems rather arbitrary that we've only done this on the second and fifth degree of the scale. Why not build a minor 7th chord upon E as well? or a major 7th chord on F? or all of the notes in the scale for that matter?

Answer 1

We do! It's just that that book doesn't...yet.

We build seventh chords on all scale degrees; the seventh chord on scale-degree 3 in major, for instance, is a minor seventh.

But beginning musicians, especially those in the popular genres, get the most bang for their buck with seventh chords on scale-degrees 2 and 5 so that they can create that nice circle-of-fifths ii7-V7-I progression.

This book is almost certainly only presenting those two seventh chords for easy clarity early on.

This text is the "basic" Harmony by Sarnecki. The book comes in three volumes (Basic, Intermediate, Advanced), and based on the Amazon description for the Intermediate volume, the seventh chords you mention are found there:

The Intermediate level addresses fundamentals of harmonization; harmonic sequences; leading-note and other diatonic 7th chords; tonicization and secondary dominants; modulation; dominant 9th, 11th, and 13th chords; the Bach Chorales; form and analysis; and melody writing.

Answer 2

In jazz and modern classical music, all kinds of 7th chords and beyond are used. But in the past, the most common 7th chords in classical music were built on the supertonic and dominant scale degrees for a very simple reason.

The ii chord and IV chords function as subdominants Mash them together and you get a iim7. The V chord and the diminished vii function as dominants. Mash them together and you get a V7 chord.

These 7th chords developed because of the shared function, but also because the m7 created a dissonance that was not so striking and out of place in the music.

Answer 3

Why not build a minor 7th chord upon E as well? or a major 7th chord on F? or all of the notes in the scale for that matter?

You are reading way too much into things: Nowhere in the content you posted does it say that 7th chords are only built on those two scale degrees. It's simply an opening exercise introducing 7th chords in two of their most important, common usages in western music.

As others have mentioned, we can and do build 7th chords on any note of any scale, and certainly the book deals with other types of 7th chords further on.

Answer 4

To complete the picture, we have major and minor triads, and to each we can add a

major or minor 7th: MM7(M7), Mm7(7), mm7(m7), mM7 (rarely used).

         key C: CM7 FM7 GM7, G7,     Dm7 Em7 Am7, DmM7 EmM7 AmM7