The lydian b7 scale

The Lydian b7 (flat 7) is the fourth mode of the melodic minor scale. It is the lydian scale with a (can you guess it?) flat seventh. In whole tones/half tones, it is:

W W W H W H W. ( W stands for whole tone, H for half tone) For example,

C lyd b7: C D E F# G A Bb

This would be the fourth mode of the G melodic minor scale, which is: G A Bb C D E F# G (if you're unfamiliar with jazz terms, mode means building a scale from another but starting on a different note, giving a different sound). As you can easily see, G melodic minor and C lydian b7 have the same notes.

Other examples:
F lydian b7 (derived from C melodic minor) : F G A B C D Eb
A lydian b7 (derived from E melodic minor) : A B C# D# E F# G

When to use the lydian b7 scale

The lydian b7 scale was introduced mainly in the bebop era, although it was used a lot earlier by certain composers. It can be played over any 7th chord, but is mostly reserved for 7th chords which don't resolve a fifth below. So for example in the II-V-I progression Dm7 - G7 - Cmaj7, the G7 is often substituted for Db7, giving Dm7 - Db7 - Cmaj7. The Db7 in this case, takes the lydian b7 scale. This comes instead of the mixolydian which would usually be played over the G7 chord. If you look at the notes in Db lydian b7, you will notice they are identical to those in G7 altered. This is the reason that if the piano player plays a G7 chord, and the sax plays a Db7 lydian (as Sonny Rollins loved to do), it sounds just great.

In the mixolydian scale, the 4th note is an avoid note. (In G mixolydian, C is an avoid note). The lydian b7 scale has the advantage of not having the 4th. Instead it has the #4, which can be played (beware, it has an unusual sound).

Should the composer wish you to play the lydian b7 specifically, the chord will usually be marked X7#11 (for example Bb7#11). In bebop, sometimes X7b5 is written. This is technically wrong, and the 7b5 usually implies a diminished scale, but what can we do, bebop musicians are sometimes idiots.

The best place for the lydian b7 is over a 7th chord which does not resolve to a fourth above (or a fifth below). So on the progression G7 - Cmaj7, the mixolydian would be most appropriate, but on, for example, Ab7 - Bbmaj7, I would use the Ab lydian b7 scale over the Ab7 chord. This is true of all SCDF chords (I7, II7, IV7, bVI7, bVII7, VII7) and with flat 5 sub chords, as the #11 of the lydian scale is the root of the substituted chord.

For example, instead of G7 - Cmaj7, we can play Db7 - Cmaj7. the lydian b7 is most fitting over the Db7 chord. And as you can see, the notes of the scale are: Db, Eb, F, G, Ab, Bb, B (technically Cb).