Musical Theory V

Seven Chords

Some of the more classically trained among you may know these as "Dominant Seventh Chords". The thing about Classical music training, is it tends to make everything much more complex than it needs to be. Jazz was the music of the uneducated masses, and although it developed into something extremely complex, emphasis is placed more on the concepts than silly teminology. For this reason, I'll be using a lot of the Jazz names when referring to chords, as they tend to be much more straighforward.

Last week, we covered major and minor triads. These are three note chords, and are fairly simple sounding. Our first introduction to four note chords will be the simple seven chord. There are many four note chord structures, diminished, augmented, suspended, major sevens, but these all build off of the basic first, third, fifth, and flat seven.

The first, third, and fifth are our triad, so this you should be familiar with. To turn a triad into a seven chord, add the note one semitone lower than the seventh note of the scale of the same name. For example, a C7 chord would be the C major triad C, E, G, plus the flatted seventh of the C major scale, Bb. Our D major triad can be turned into a D7 chord by taking the seventh note of the D major scale, C#, and lowering it by one semitone to become C before adding it to the D major triad. Therefore a D7 chord is D, F#, A, C.

Minor seven chords work in a similar fashion, only you take the seventh note of the major scale of the same name and lower it by a semitone. This means that the only difference between a D7 and it's minor counterpart, D-7 (the negative sign denoted minor) is the third.

D7 : D, F#, A, C
D-7 : D, F, A, C

Quiz Question:
Why would a seven chord be referred to as a "Dominant Seventh" in Classical Music Theory?