How is the Italian sixth chord constructed?

The basic formula of the Italian sixth chord, which is a diminished seventh augmented fifth chord , is composed using four notes. One starts with any basic triad, in either root position or first inversion; then one adds, a half step below the lowest chord tone, the seventh tone of the scale on which the chord is built.

For example, to build an Italian sixth chord on C, begin with C major or C major sixth chords (C–E–G or E–G–C), and add B-flat. The resulting four notes constitute the Italian sixth chord: B-flat–C–E–G.