bII7

bII7, written as with a flat-two seven or ♭II7, is a dominant seventh chord built on the flatted second scale degree of a key. In a major key, this means constructing a major triad on the b2 scale degree with a minor seventh added. For example, in C major the bII7 is Db7, which contains the notes Db–F–Ab–Cb (Cb enharmonic to B).

Function and usage: The bII7 commonly functions as a predominant that leads to the tonic (I), producing a distinctive Neapolitan-like color and strong resolution via chromatic voice-leading. A typical resolution in C major would move Db7 to C major, with voices moving by semitones (for example, Db→C, F→E, Ab→G, Cb→C). In jazz and popular music, bII7 can also serve as a tritone substitute for the V7 of I; in C major, Db7 can substitute for G7, yielding a similar dominant-to-tonic pull.

Relationship to related chords: The key-related Neapolitan concept is closely connected, as the basic Neapolitan chord is the bII major triad, and the bII7 adds the seventh for additional color and stronger resolution. The spelling of the third and seventh notes (and sometimes the use of enharmonic spellings) is chosen to preserve smooth voice-leading into the subsequent chord.

Scope and variants: While most often discussed in tonal harmony in major keys, bII7 also appears in minor-key contexts, where it retains its chromatic predominant function, though the exact resolutions may vary with modal or melodic considerations.