# what scale is used for solo over bIIIMaj7 when approaching II-7?

Three ways are available when approaching II-7 via bIII. bIIIMaj7, subV/II, bIII full dim.

I blindly thought that Ionian scale is used for solo over bIIIMaj7, but apparently Lydian is used.

Can anybody explain why?

Because the note A is diatonic in the key of C major. It is also shared with the following II-7 chord. This is the raised 4th degree which distinguishes lydian from ionian.

In a major-seventh chord, the natural 11th (in this case, the note Ab in an EbM7) sounds particularly bad. The #11th is often used instead. This is true in any key.

user1079505's answer showing that the #11 is a member of C major is another good reason.

In this particular case you can superimpose the two seventh chords and that just happens to give you a diatonic scale.

C:
bIIIMaj7 =   Eb   G   Bb   D
II-7     = D    F   A    C

Ordered by step from C = C D Eb F G A Bb C


Nominally you get C dorian. The bIIImaj7 chord would be a borrowed chord in C major so that kind of C major/C dorian mode mixing makes sense.

This isn't necessarily the best approach to improvisation, but seemed worth mentioning, especially if the approach is to match up some "scale" of pitches to a chord or chords.

FWIW, there are a number of scales that can be describe as the superimposition of two chords. The octatonic scales come to mind, being superimpositions of two diminished seventh chords.

I don't think you need to worry too much about the Ab versus A natural, especially if the argument is about A natural's diatonic identity in C major. Firstly, the chord movement is chromatic. Secondly, it seems a pretty dogmatic attachment to diatonicity when the context is jazz. Either pitch could work if the melodic sense is solid. I'm I supposed to believe that the Ab (enharmonically G#) could not be employed as the lower auxiliary to the A, the chordal fifth of Dmin7? Of course it could!