Chord ii7 is also known as the supertonic 7th chord. It’s built from the supertonic triad, plus another third added on top: the 2nd, 4th, 7th and 1st notes of the scale.
In C major, the supertonic 7th chord contains the notes D, F, A and C.
In major keys, the supertonic 7th is a minor triad, plus a minor 7th above the root note. It is written as ii7.
In minor keys, the supertonic 7th is normally built from the harmonic minor scale. It is a diminished triad, plus a minor 7th and is written as ii°7.
In A minor, the supertonic 7th chord contains the notes B, D, F and A.
The supertonic 7th is frequently seen just before a perfect cadence, or before a cadential 6/4, for example in this Bach Chorale (no.95), in Bb major. Here, ii7 is C-Eb-G-Bb, it is used in first inversion here (which is very common, as the bass (lowest part) rises by step to Va or Ic).
In this G minor chorale (no.100), chord ii°7 is A-C-Eb-G. The progression is the same as in the previous example.
