Diatonic- notes in the specified key.
Chromatic- any note, regardless of key.
Atonic- a feeling that the music possesses no key center.
Western music is diatonic.
The Ionian Greek tribe won out, at least musically, and their musical taste defined Western music. The Ionian Mode is the Major Scale… and is what the VAST majority of Western music is based upon… as far as what sounds “good” to Western ears….
Other cultures embrace different tonal scales, but to the Western ear… those sounds… strange….
Harmonicas are probably the most common musical instrument played in the Western world. Most harmonicas are diatonic, meaning that they come in a key. As one becomes more adept, and learns how to bend certain notes on the draw… one can play a modified “Blues” scale that has some notes outside the standard diatonic scale of the key, but most people never progress to second or third position playing on their harmonica.
Harmonicas also come in the Chromatic variety, which are capable of playing all twelve tones in the eight note scale.
The Ionic Mode or diatonic scale is: whole-whole-half-whole-whole-whole-half steps… and then repeat….
That translates to: I, ii, iii, IV, V, vi, vii0….
The capital letters are Major chords, the uncapitalized letters denote minor chords and the 0 means diminished.
A Major chord is three notes a major third of the scale apart and define what sounds “good” to Western ears.
I is Tonal. IV is Subdominant. V is Dominant… and almost forces resolution to the Tonal I….
Minor chords have a flatted or minor third between the Major fifth bookends. Minor chords sound “sad” or “haunting” to Western ears.
A diminished chord flats the fifth because that needs to happen with the viith chord because the scale fits twelve half-tones into eight steps… and the last step… from vii to I… is only a half-tone…. Which usually sounds dissonant… or adds tension… is hard to fit within the key….
If one begins on middle C… which is considered the tonic center of the piano keyboard… then the progression is C, D, E, F, G, A, B… or all of the white keys…. You can look at the keyboard and visualize the Ionian Diatonic Scale starting at C.
If you want to know what a mode is… simply look at the white keys and start on a different note….
Start on the Dominant Vth and you get G Mixolydian, which is different from G Major because the 7th step… which would be F#… is now F instead… leaving a whole step instead of a half-step at the end of the scale. Mixolydian modes are considered Major scales….
Start on vi and you get the Aeolian Mode or Pure Minor or Natural Minor Scale, which is A minor; however, this scale introduces some harmonic difficulties… because to the Western ear… ascending on a Natural Minor Scale sounds fine… but descending sounds… out of tune…. Descending sounds better in what is called the Harmonic Minor Key….
So… i, ii0, bIII, iv, v, bVI, bVII (where b means flat)….
Then… i, ii0, bIII+, iv, V, bVI, vii0 (where + means augmented or sharp 5th)….
The point is that a lot of different scales exists… so the paths through any particular set of notes can be very varied…. Some paths sound better than other… and what music you were raised listening to defines what sounds “good” to you….
Some paths are more interesting tonally than other paths….
That’s why they make chocolate and vanilla….
Harmonic Rhythm… or how the notes and chords are tied together in SpaceTime… the frequency of chord changes… dictates the pace… and defines the mood… or the piece….
So… ideally… the Harmonic Structure… and the Harmonic Rhythm… should work together with the Melodic and Lyrical components to support one another… or help create dynamic tension at different parts of the work….
Of course… some Composers are vehemently opposed to the use of music to drive emotion….