If we extend the chord triad by giving it a fourth note, we get 7th chords. I will not go any farther than charting 4 note chords. Applying chords to music To apply chords to music, first find out the key of the song. Then by harmonizing the corresponding scale, or simply using the number system, one can find all of the chords which work for a given key. But try it anyhow, it may work in the song even though it doesn't fit the key.
Keys are to be thought of as guidelines.
When the notes of the scale go into another octave, the numbers continue. Naming Chord Triads Earlier, I demonstrated how to find the name of chord triads using intervals. The chart below illustrates how a chord name can be found by examining the numbers of the major scale in which it is derived from. Chords A chord which has its third either raised to the 4th or lowered to the 2nd scale degree. There has been some confusion about suspended chords.