The Supertonic Seventh Chord: 

Like the supertonic triad, the supertonic seventh chord is commonly used to precede

the dominant.  In fact, in the "full cadence," the progression I-ii7-V-I (most often ii six-five) is

far more common that the I-IV-V-I or the I-ii-V-I progression.  In addition, the supertonic

seventh is usually the preferred choice for a dominant preparation elsewhere in the phrase. 

The supertonic seventh chord is a minor seventh chord (mm7) in a major key (ii7), and a

half-diminished seventh chord (dm7) in a minor key (ii7).  (The ii six-five is sometimes called

an "added sixth" chord in that it sounds like a triad with an added 6th scale degree.)

Characteristics of the Supertonic Seventh Chord:

The approach to the seventh is commonly made by way of a suspension figure. 

Since the seventh is the tonic of the key, such an approach is very easy.  The chords likely

to precede ii7 are vi and IV, both containing the tonic note of the key.

The first inversion of this chord is used far more often than other positions, though all

are useful. Like the ii6 and IV triads, the bass note of the ii six-five is scale degree 4,

providing an excellent preparation for the dominant tone (5) in either V or tonic six-four.

The ii six-five is found in the most common of the passing six-four chord

progressions:  IV6 - I six-four - ii six-five.

The seventh is commonly held over to become the root of the tonic six-four.  Since

the root of I six-four is a dissonance (a fourth above the bass), the seventh of ii7, simply by

standing still, becomes another dissonance, which then resolves down by step, taking care

of both dissonances.