All melodies, all backing chord part, ultimately every Music work has a harmony part.
Chords are the building blocks we use to explain the harmony.
Harmony is based on the way what we play sounds related to the other elements of that song.
Chords are builded from overlapping notes in thirds.
We can build chords using overlapping minor and major thirds. The triad is composed of the first three notes of that cluster.
This chord can be reversed in two other ways. Any other combination is possible and it is the base of all the chords and melodies. The chord inversions are the various dispositions of the notes in a chord.
Tonality Keys are essential for building chords. Flats and Sharps in the Key indicate the notes that we must alter.
The three sharps in the A Major, F# minor Key
As well, Tonal degrees are basic to write songs, because there are only three harmonic areas: Tonic, Subdominant and Dominant.
All the scale notes are related to each other and also with the Tonic of this Harmony Area.
If interval between the seventh grade and the Tonic is a Minor Seventh, then it is called SUBTONIC.
Each Key has three harmonic areas. In the first, fourth and fifth grades are tonal chords of Tonic, Subdominant and Dominant, respectively.
With these three chords we have got all notes arranged and some simple tunes on which we can compose melodies.
We use the English (or American) nomenclature(Symbol Notation) for naming the chords. Knowing of the intervals is essential to this system.
This nomenclature is used in harmony. The chords are based on the musical notes and intervals, receiving from them their names. With this system we build chords.
Knowing how the chords are created is not enough to know how to play them. Musicians must learn to play in different styles because they not only have to learn to play the chords, but they must know how to play that progression in a particular song in a particular Style.
A chord sounds different in every type of Music if that that chord is played according to a specific convention.