Playing 2 notes the one which is obtained he is bicordo cioe’ turning out of the concomitanza of two sounds.

Adding a further note, supposing as an example, to play three sounds D-F#-A simultaneously (with D the lower) what it is obtained is a chord: three (or more) sounds played at  the same time.

Modalities  which such sounds come chosen are defined by  harmony, which characterizes the rules to use in the construction/combination of the sounds (chords).

The lowest sound in a chord is said Root (fundamental).

In the previous  example (D - F# - A triad), D is the root. Now, given the half-tone the minimum distance between 2 sounds (ex: C - C#), adding to the D (root) an interval of major third (4 half-tones) we get the 2nd sound, F#. Still adding from the D (root)  a perfect interval of fifth (7 half-tones) we have the 3rd sound, A.

The RE-FA#-LA triad (or chord)  thus is formed from two intervals: a major third and a  perfect fifth, and it can be considered as further shown, as the basic structure of  major chords.

In general a chord is formed from a series of intervals beginning from the root. Chosen the root sound, others can be added, away frm it a specific interval according to the reference scale.

So, a thus constructed chord comes defined TRIADE (3 notes) , and allows us to build the basic structure of every kind of chord, according with follows:

Major Chord Triad (M)

Root - Major 3rd (4 semitones from the Root) - Fifth (7 semitones from the Root  Ex. C - E- G)

Minor Chord Triad (m)

Root - minor 3rd (3 semitones from the Root) - Fifth (7 semitones from Root Ex. ES. C - Eb - G)

Diminished Chord Triad (d)

Root - minor 3rd (3 semitones from the Root) - Fifth Diminished (6 semitones from Root Ex. C - Eb - Gb)

Augmented Chord Triad (M+)

Root - Major 3rd (4 semitones from the Tonic) - Fifth Augmented (8 semitones from TonicES. C - E - G#)

Adopting from now on:

1 - The roman numeration for the degrees of the referred scale and

2 - The jazz notation,

the interesting thing once at this point is that, starting from the Root (now 1° degree), ascending along the Major scale, the progression of triads built on each note is always the same:

I - Major (M)

II - minor (m)

III - minor (m)

IV - Major (M)

V - Major (M)

VI - minor (m)

VII - diminished (d)

The most important Triads are those built 1°, 4° and 5°, on which the V-I cadenza refers, sequence that incluced the concept of Tension-release widely used in jazz harmony, through the frequent use of the TurnAround, a fundamental chords progressions in the harmony jazz, that lead back to the Root Triad (chord):

Ex.     I    VI      II      V –>  I° M  -  VI° m  -   II° m  -   V° M

using Triads    –> C Major   A minor     D minor    G major –>   C (M)

The following table reassumes Triads for the Major scale (in the key of A):

What seen for the Major scale Triad is therefore applicable also for the Minor scales.

Therefore if proceeding ascending along the degrees of a Major scale we have the sequence M - m - m - M - M - m - m - d above seen, we obtain Triads for minor scales , proceeding as well:

Once get used on a keyboard with these concepts, we can expand our analisys and consider 4 notes Chords.