The seventh chord is one of the most frequently occurring sounds in contemporary music. Funk, blues and jazz music are all replete with the dominant sound. It is most commonly notated with a 7 next to the letter, i.e. C7. This should not be confused with other notations such as M7 and m7, which represent major and minor, respectively.
Building the Chord
The only difference between major and dominant chords lies in the interval of the seventh. They both are built upon major triads, but dominant seventh chords have an interval of a minor seventh between the root and seventh. For example: CM7 has an interval of a major seventh between C and B. C7 (dominant) has an interval of a minor seventh between C and Bb.
It is readily seen that since the difference between B and Bb is a half step, the dominant chord is smaller. This is a defining characteristic that distinguishes minor from major. Major intervals are larger, while minor intervals are smaller. Another important characteristic to be aware of is the tritone that occurs within the dominant chord. Take C7, for example:
C7 is composed of the notes: C E G Bb. From C to E is a major third. E to G is a minor third. G to Bb is another minor third. Compare this to the makeup of a major seventh (C E G B). In a dominant chord the interval between E and Bb is a tritione. This is a very dissonant interval and creates much tension within the chord. This is what gives dominant chords their characteristically funky sound.
Chords Within the Diatonic Scale
The term dominant refers to the position of the chord within the diatonic scale. Dominant seventh chords naturally occur in the fifth position in both major and minor scales. Additionally, in the minor mode, they also occur in the seventh position. Take, for example, the C major scale with no accidentals:
C D E F G A B C.
The dominant seventh occurs in the fifth (V) position of the scale at the G note. Playing the C scale starting from G results in a mixolydian scale. Take a closer look at the notes which make up the G7 chord in C major: G B D F. Again, there is a major third between G and B, a minor third between B and D and another minor third between D and F. The reader will note the tritone between B and F.
Extensions and Substitutions
The versatility and diversity of dominant chords stems from all of the different extensions and alterations that can be applied. Since the nature of the triad is major, that is not altered. Instead, the interval of the fifth can be sharped and flatted (in C7, this would be G). Additionally, other intervals such as ninths, elevenths and thirteenths can be added. These intervals can also be altered to create the desired dissonance.
The following is a list of commonly used extensions on C7: C9, C7b9, C7#9, C11, C7#11 (also written as C7b5), C7b13, C13. The common denominator here is the C major triad. All of the chords listed here contain only one alteration, but multiple extensions are also common. C7b9#11, C7#9b5 and C7b5b13 are possibilities.
C7b9#11 contains the notes C E Gb Bb Db. While the player might choose to omit one of these notes, notice that within this chord exists another chord. The notes Gb Bb Db form a Gb (F#) triad. Playing F#7 instead of C7 is called substitution and is a valuable tool to use since it can free up stagnant harmonies.
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