Harmonic minor keys are used when writing chords ... the triads in C major are as follows: Triads can be indicated by Roman numerals of the degrees of the scale. In the jazz styles of Gershwin ...

The first 7th chord most of us learn is called just that: C7, G7, A7, etc. It seems reasonable to assume these consist of the root, 3rd, 5th and 7th of the scale ... b7th (C) and major 3rd ...

Chords are numbered in Roman numerals. For example, in the scale of C major, the chords are numbered like this: An added 6th is a major triad with the addition of a 6th note above the root.