In classical music, and in rock, there
are three principal chords: the major triad, the minor triad, and the dominant seventh. So for example, in the key of C, the major triad consists of the root, which is C, the major third, which is E, and the fifth, which is G.
Or played an octave lower, so that I can play the scale in the middle of the piano, C, E, G. The scale that goes with it is the well known “do, re, mi, fa, so la, ti, do” scale,
which we shall call the Ionian scale or the Ionian mode. A mode is simply a scale which is rooted in a different position. So the Ionian mode is.
The minor triad consists of the same thing but with the flattened third. In other words, it’s the root C, the flattened third, E flat, and the fifth G. And with it is a scale that starts off with the major third flattened.
But then there is an ambiguity when we get to the sixth and seventh steps of the scale, because these may either be flattened or naturalised. For example, we may either use A flat or A – so there are two possibilities at the sixth level – B flat or B. – so there are two possibilities at the seventh level. 2 times 2 is 4. So there are four possible scales depending on which of those choices we make for the sixth and seventh. OK, although there is some ambiguity, probably the most used scale in classical music is the harmonic minor. And the harmonic minor consists of the flattened sixth and the natural seventh.
The flattened sixth, which is A flat And the natural seventh, which is B natural.
And the third chord consists of C triad,
added to which you have the flattened seventh B flat, in the key of C. This is called the dominant seventh. This chord is the dominant seventh because C – the root – is the dominant of F.
It is a passing chord. It moves from C7 to F.
So a dominant seventh is a passing chord and it takes you from the dominant to the tonic, which in this case is from C7 to F. And with it the scale — I like to think of all scales relative to the Ionian scale, relative to the major scale — and it is simply the major scale with a flattened seventh –
all the white notes apart from B flat. In fact, the notes are the same as the scale of F major, but they’re rooted on C.
Hence, it’s called a mode – a mode of F. But we’ll discuss that in more detail later on.
OK, the three chords are: the major triad.
and the Ionian scale which goes with it. The minor triad
and the harmonic minor which goes with it. And then the dominant seventh – the passing chord –
and we call that the C seventh scale, namely the major scale with the flattened seventh.