TONALITY
—DISTINCTION—
Tonal
music is centered around a tonal center, the tonic, which also determines a key. The vast majority of western music is tonal.
Example:
Carpenters:
"(They Long to Be) Close to You"
Modal
music revolves around a scale. In Europe, modal music was the predecessor of the more polyphonic tonal music. Modal music is still practiced by various traditional cultures and can be found in some jazz pieces.
Example:
"Tribulationem et dolorem"
By Carlo Gesualdo
Atonal
music uses tone rows of the chromatic scale to express abstract ideas. Atonal music aims to avoid a center.
Example:
King Crimson:
"Moonchild"
—KEY—
Major key
a.k.a. major mode
Music that uses the first tone of the major scale as its final resolution tone.
There are 12 possible major keys, one for every note on the chromatic scale.
Example:

"Sonate Nr. 16 C-Dur" By Wolfgang Amadeus Mozart
Minor key
a.k.a. minor mode
Music that uses the sixth tone of the major scale as its final resolution tone.
There are 12 possible minor keys, one for every note on the chromatic scale.
Example:
The Dave Brubeck Quartet:
"Take Five"
—FUNCTION—
Degrees of the MAJOR scale can fulfill specific functions.
I
a.k.a. Tonic.
The 1st degree on the major scale is the center of any tonal music. The final resolution tone or harmony is built upon this "expected" degree that ends all the tension. A I cannot substitute a IV or a V.
Example:
I-V-IV.
The Who:
"Won't Get Fooled Again"
V
a.k.a. dominant.
The 5th degree on the major scale is next in importance to the I. A dominant harmony creates tension that urges for a tonic harmony to resolve it. A V cannot substitute a I or a IV.
Example:
I-IV-V.
Ramones:
"Blitzkrieg Bop"
IV
a.k.a. subdominant.
The 4th degree on the major scale is next in importance after the V. A sub-dominant can function as a calm anchor and lets the music progress more freely than a dominant harmony. A IV cannot substitute a I or a V.
Example:
I-IV.
Son House:
"Death letter"
vi
a.k.a. submediant.
The 6th degree on the major scale plays a less significant role. It usually comes after a I and iii, and before a ii, IV, V AND/OR can substitute a I. (The vi is the i in minor mode.)
Example:
vi-I-vi.
Lykke Li:
"I Follow Rivers"
ii
a.k.a. supertonic.
The 2nd degree on the major scale plays a less significant role. It usually directs the harmonic motion immediately towards the V AND/OR can substitute a IV.
Example:
vi-IV-I-ii.
Aerosmith:
"I Don’t Want To Miss A Thing"
iii
a.k.a. mediant.
The 3rd degree on the major scale plays a less significant role. It usually directs the harmonic motion towards the V, rarely progressing directly AND/OR may substitute a V.
Example:
IV-V-iii-IV-(IV)-V-iii
Cyndi Lauper:
"Time After Time"
vii°
a.k.a. leading tone.
The 7th degree on the major scale plays a less significant role but has unusual characteristics (dimished chord). It usually leads to the I or III as a passing chord AND/OR can substitute a V. It's almost non-existent in popular music.
Example:
iii-ii-vii°.
Duologue:
"Endless Imitation"
Secondary functions
Every function is relative, this means that other functions can have secondary functions, like a v of a ii.
Example:
V of a vi.
Radiohead:
"Creep"
—CADENCES—
A progression of chords that creates a sense of resolution or pause.
Perfect cadences
a.k.a. authentic cadences,
are ending with the harmonies V-I.
Example:
Fools Garden:
"Lemon Tree"
Plagal cadences
a.k.a. amen cadences,
are ending with the harmonies IV-I.
Example:
Weezer:
"Say It Ain't So"
Half cadences
a.k.a. imperfect cadence,
are ending with V.
Example:
Don Mclean:
"American Pie"
Deceptive cadences
a.k.a. interrupted cadences,
are ending with the harmonies V-vi instead of V-I.
Example:
Doobie Brothers:
"Long Train Runnin"
Picardy third
a.k.a. Tierce de Picardie.
The tonic chord at the end of a section, that is in a minor key, is played as a major chord.
Example:
Lionel Richie:
"Hello"
—MODULATION—
Changing the key for artistic expression.
Tonicization
happens if a new I is established (temporary). If held long enough it leads to a modulation. Tonicization can be achieved for instance through the use of secondary dominants.
Example:
Phantom Planet:
"California"
Common-chord modulation
a.k.a. pivot chord modulation.
Closely related keys share so-called pivot chords. The common-chord modulation uses these chords to move in-between the keys.
Example:
No Doubt:
"Don't Speak"
Common-tone modulation
At least one note of the modulating chord gets sustained or repeated while the another notes get replaced: The last chord of a key is also the first chord of a new key.
Example:
Oingo Boingo:
"Violent Love"
Enharmonic modulation
The enharmonic modulation uses the ambiguity of the dominant seventh chord (lowering a note of a diminished seventh chord a semitone leads to a dominant seventh chord vice versa): I-V-V-I whereas the second I belongs to another key, therefore leads to a modulation.
Example:
"Sonata Romantica"
By Nikolai Karlovich Medtner
Chromatic modulation
Change of key is achieved through moving a note (in a chord) a semitone rather than with a pivot chord.
Example:
Eric Clapton:
"Tears in Heaven"
Sequential modulation
a melodic phrase or a chord progression gets repeated at a different pitch.
Example:
Astrud Gilberto & Stan Getz:
"The Girl from Ipanema"
Parallel key modulation
Chaning between minor and major mode but keeping the scale.
Example:
The Turtles:
"Happy Together"
Abrupt modulations
a.k.a. direct modulation. Change in key can also happen abruptly after a cadence.
Example:
Ray Charles:
"Georgia on My Mind"
The truck driver's gear change
A shift upwards by a semitone, typically at the end of a section.
Example:
Michael Buble:
"Feeling Good"
—HARMONIES—
Borrowed chords
a.k.a. modal interchange,
If a major key shares a tonic with a minor key then they are parallel keys. Chords from the parallel key are used to give music more color.
Example:
I-♭III-IV
The Dandy Warhols
"Not Your Bottle"
Contrast chords
a.k.a. Gegenklang,
are substitional chords. They are formed by altering only one note so that a minor triad becomes a major triad, or a major triad becomes minor triad.
Example:
The Warlocks:
"Above Earth"
Voice leading
is the tie between the consecutive pitches of simultaneously moving voices in the chords. Good voice leading creates a notion of a smooth auditory stream.
Example:
"An der schönen blauen Donau"
By Johann Strauss
Augmented sixth chords
a.k.a sixte ajoutée,
are containing the interval of an augmented 6th and are used often in classical music either as a Italian 6th,

as a French 6th,

or as a German 6th.

and are treated with voice leading or regarded as a secondary dominant, predominant or as a modulating chord.
Example:
The German 6th.
"Requiem"
By Michael Haydn
Neapolitan chord
a.k.a. Neapolitan,
is a major chord built a semitone under the ii. They are treated with voice leading or regarded as a secondary dominant, predominant or as a modulating chord.
Example:
Notice the last harmony.

"Klaviersonate Nr. 14" By Ludwig van Beethoven
Approach chord
a.k.a. chromatic approach chord,
are chords a seminote higher or lower from the destination chord.
Example:
Jimi Hendrix:
"The Wind Cries Mary"
Tristan chord
There are books addressing only the topic of this chord(progression). It is named after its use by Richard Wagner.
Example:
"Tristan und Isolde, Act 1"
By Richard Wagner
Passing chords
are outside the harmonic framework of a key and in-between two chords inside the harmonic framework of a key.
Example:
Miles Davis:
"Bitches Brew"