What is the function of the ii chord?

Subdominant, also called predominant. It leaves the tonic and normally moves on to V, as in ii–V–I, so it prepares the dominant.

Is the vi chord a tonic?

It shares two notes with I, so it can stand in for the tonic and sounds like a gentler rest. That is why V going to vi is a deceptive cadence: the ear expects I and gets another chord of tonic function.

What is the difference between ii–V–I and I–IV–V?

Both run through the same functions. In a major key, ii–V–I leads from ii, a minor chord with subdominant function, through V to I, the standard cadence of jazz and classical music. I–IV–V starts at home with a major IV, is the backbone of rock and blues, and usually goes on to I.

Harmonic Function: Tonic, Subdominant and Dominant

Harmonic function is the job a chord does in a key: the tonic is home, the subdominant leads away, and the dominant pulls back. In a classical phrase the usual path is tonic, subdominant, dominant, tonic.

What are tonic, subdominant and dominant?

Chords are not interchangeable bricks. In a key each one has a tendency, and the tendencies come in three kinds. A tonic chord is stable: the music starts there and rests there. A dominant chord is tense and wants to resolve to the tonic. A subdominant chord is neither at rest nor urgent; it simply leaves the tonic and sets up the dominant. Many teachers call this middle group predominant, because it comes before the dominant.

I–vi–ii–V–I in C major, each chord labeled with its function: T (tonic), SD (subdominant), D (dominant).

Function	Chords in a major key	What it does
Tonic (T)	I, vi, iii	rest: where phrases begin and end
Subdominant (SD)	IV, ii	leaves home and builds toward the dominant
Dominant (D)	V, vii°	tension that wants to resolve to the tonic

The usual path in a classical phrase is tonic, then subdominant, then dominant, then back to the tonic. Going the other way, from dominant back to subdominant (V–IV), sounds like reaching the door and turning around, and is rare in such phrases. Blues and rock use it freely: measures 9 and 10 of the twelve-bar blues are exactly V7 to IV7. Chords of one function can stand in for each other: ii in place of IV, and the phrase keeps its shape.

Why does function explain cadences?

A cadence is the end of a phrase. The strongest one, the authentic cadence, works because the last two chords are a dominant moving to a tonic: the tension is released at the moment the phrase stops. A half cadence stops on the dominant, so the tension stays and the listener waits for more. Function also says what the four cadences are doing: authentic is dominant to tonic, plagal is subdominant to tonic, half is anything to dominant, and deceptive is dominant to a different chord with tonic function, vi.