Why are some chords called 7th's?

How many different kinds of 7th's are there in music?

These questions come up every once in a while, when I'm teaching guitar lessons to my students. There seems to be a slight confusion as to what a “7th” chord actually is. All of it simply has to do with basic music theory applied to common chord structure.

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Why is it called a 7th chord?

The 7th does not mean that there are seven notes in the chord, or that you are arbitrarily choosing some random 7th note in a scale to define the chord. The seventh note in the diatonic scale is the 7th.

In a previous article, I mentioned "The Sound of Music". In this classic movie, Julie Andrews' character takes the children on a hike into the mountains where she sings the infamous "Do Re Mi" song. This is something most children learn to sing in preschool or kindergarten. (Or at least they used to when I was a kid).

"Do, Re, Mi, Fa, So, La, Ti, Do" - Do you see the "Ti"? That is the seventh two-letter word in the song. Each one of these two-letter words has its own corresponding Roman numerical value:

Do = I, Re = II, Mi = III, Fa = IV, So = V, La = VI, Ti = VII... and Do again, as it is considered the "I" to start the next octave.

When you learn chords on the guitar, some of the very first chords you learn are major and minor chords. These chords are called "Triads". Triad means three. There are three notes in a triad major and a triad minor chord. The structure of a "7th" chord adds one more note to the triad. It just so happens to be the "seventh note" in the scale.

However, there are actually two separate 7th's in music. One is the "Major 7th" and the other is the "Dominant 7th". These are two different intervals in music.

For example: 

The "C" major chord structure requires the notes C, E and G. 

The "C Major 7" chord structure requires the notes C, E, G and B. "B" is the 7th note in the key of C major.

The "C (dominant) 7" chord structure requires the notes C, E, G and Bb. "Bb" is one-half step below the major 7th note. 

So, in the key of C major, the B is the Major 7th of C, and the Bb is the dominant 7th of C.

What about other triads?

Let's say the song we are playing requires a minor 7th chord. This chord is simply a minor triad with a dominant 7th added to it. You won't normally see a chord written out as a minor "dominant" 7th. It is assumed that the dominant 7th is the default 7th of the minor 7th chord, unless otherwise indicated. Yes, there is such a thing as a minor chord with a major 7th. It's a weird sounding chord that usually needs to be in context with the chord progression to make sense. By itself, it's not very pleasing to the ears.

How to hear the differences between major and dominant 7th chords.

It's actually pretty easy to hear the differences between these two types of seventh chords. Major 7ths sound pleasing to the ears. Some (as I do), refer to major 7ths as "jazz" chords. They have a relaxing, harmonious sound to them. Whereas dominant 7th chords are a little harsher as they sometimes sound somewhat "unresolved". These types of seventh chords are used mostly in rock, country and of course, the blues music.

Diminished, half-diminished, augmented and suspended chords all normally use variations of dominant 7th in them. Again, these are context and resolving type chords that sound good in a progression that makes sense and is pleasing to the ears.

Recommendations to practice:

Try practicing and learning the following chords and add them to your chord vocabulary. You can find them on many of my previous guitar lessons articles here on SoundHole.

FM7, F7, Fm7

CM7, C7, Cm7

GM7, G7, Gm7

DM7, D7, Dm7

AM7, A7, Am7

EM7, E7, Em7

BM7, B7, Bm7

Hopefully, this lesson has helped you better understand the differences in types of "7th" chords. If you like this lesson, please consider a paid subscription to help support my work in writing these articles. Comment below and please share. Thanks, Dave.