Scales Over Diatonic Vs Non-Diatonic Chords

Question by Alessandro
(Napoli, Italy)

Mike, your website is fantastic, and the way you explain things too.

In order for you to understand my question easily, I'll immediately start with an example:

We are in the key of A minor and our chord progression is simple: Am / Dm

In this case, as I learnt from you, I would simply use the C major scale, emphasizing the chord changes by playing the opportune chord tones at the moment when a change happens.

Now suppose we have this common rock substitution: Dm becomes Dmaj.

We are still in the key of Am, but we now have to handle a non-diatonic chord: Dmaj.

How can I improvise over it?

I suppose that the best way to improvise over a non-diatonic chord is using the approach you explain in your "How to solo over (single) chords".

Based on what I read in that post, I could use for this major triad (Dmaj), just an example, a D Lydian scale.

But my question is: since D Lydian is made of the notes D, E, F#, Ab, A, B, C# and we are playing in the key of Am, don't we risk to produce dissonant sounds when we use F#, Ab and C#, which are notes not contained in the Am key?

It was just an example, so I don't want to focus specifically on the case we have considered... I want to focus, instead, on this general problem:

If we use over non-diatonic chords the approach "per single chords" you explained in "How to solo over (single) chords", don't we risk to choose scale options with too many notes outside of the key. So... don't we risk to create dissonances? I don't know: maybe certain notes sound good even though they are not part of the key we are playing in?

Thanks very much.

Scale Vs Key

Alessandro, this is a very well thought out and well written question that many intermediate players raise, once they've got their heads around the diatonic, "single scale" approach.

First, I want you to clarify in your own mind the difference between scale and key.

If you are already clear on this difference, you will understand that a scale does not have to be part of the implied key in order to work over certain chords in a progression.

Take this example:

Dmaj / Bm / Em / Bb7

Most musicians would consider this to be a D major key progression.

However, while the D major scale would work over the first three chords, there's a rebellious Bb7 chord that takes us outside the diatonic key signature.

I used this example in my lesson on the lydian dominant scale, making that scale a strong choice over the Bb7 chord.

Why? Because lydian dominant has what I call an "outside" function.

Lydian natural (the standard lydian mode) also has an "outside" function over maj7 chords that reside outside the originally implied key signature.

All it means is we need to move back into the original scale when the progression changes back into diatonic territory.

So the risk of dissonance you mention is not an issue, as long as we can isolate the rebellious "outside" chords (both by ear and on paper) and switch to the appropriate scale over those chords.

Dissonance occurs when simultaneously played notes do not harmonise. It has nothing to do with key.

For example, over the progression Am / Dm / Em the natural scale choice would be A natural minor, as this is a standard i iv v minor key progression.

But there's nothing stopping you from playing A Dorian over Am, D Dorian over Dm and E Dorian over Em (although it would be a brave move!)

That's one-scale-per-chord and, although the progression is still in the key of A minor, our scale choices, although unconventional, do not sound unpleasantly dissonant for deviating from the "natural" scale choice (A natural minor).

In fact, such deviations can often turn a very middle of the road musical expression into something more beautiful and emotive.

So remember, key does not determine dissonance, inharmonious notes do! Choose your scales/notes based on this principle.

Is it Really Non-Diatonic?

You should ask yourself this question whenever you come across two or more chords that appear to be outside the natural major or minor scales.

Taking your example of a D major key substitution, Alessandro...

Am / Dm becomes Am / Dmaj

These are, in fact, both diatonic movements.

Am / Dm implies A minor.

Am / Dmaj implies G major. Let me explain...

Am / Dmaj is a ii / V movement. So you could see this as an "unresolved perfect cadence" that, given its melodic conclusion, would resolve to the I, which would be G major in relation to the two chords.

Try playing the G major scale or any of its modes (e.g. A Dorian) over this movement and you'll hear it sounds perfectly natural.

So although you've substituted Dm to Dmaj, all you've done is create a shift in key signature (A minor to G major).

I understand you didn't want to dwell on specific examples, but I felt it was important to clarify that.

Let's take another example...

Amaj / Dmaj becomes Amaj / Gmaj

Now, you might think "Gmaj is not part of the harmonized Amaj scale", and you'd be correct:

Amaj - Bm - C#m - Dmaj - Emaj - F#m - G#dim

But Gmaj IS part of the D major scale:

Dmaj - Em - F#m - Gmaj - Amaj - Bm - C#dim

Look at the relationship between the two chords, Gmaj and Amaj - they are the IV and V chords respectively of the D major scale!

This means what appears to be a non-diatonic substitution is actually a shift into a different diatonic key.

Being able to identify these chord relationships takes time, but it's worth it because, when you can recognise them, you'll soon realise that, actually, most progressions ARE diatonic, even if they hang around chords other than the I/tonic of the scale.

The Scale You Use Helps to Determine the Chord Type

Going back to your example of using Lydian over Dmaj in the progression Am / Dmaj, what you're essentially doing here is extending the Dmaj chord harmony from a triad to a Dmaj7 chord.

Why? Because Lydian contains the intervals of a maj7 chord!

Maj7 chord = 1 3 5 7

Lydian = 1 2 3 #4 5 6 7

Even though the notes of D Lydian are not part of the A minor key signature, this does not matter, since we're only using D Lydian over Dmaj - a perfectly compatible pairing.

Try playing the chords Am / Dmaj7 - it sounds quite eerily beautiful, and this is the harmony you're implying by using Lydian over Dmaj.

Of course, the #4 (augmented 4th interval) of Lydian is also part of this chord-harmony extension, the full lydian chord being a maj7#11.

It's great that you'd even consider using the Lydian scale so unconventionally (at least outside jazz!) because as a creative musician your ears need to hear it to know how beautiful (or ugly) it sounds over that particular chord.

And that's the bottom line here - experiment and, if it sounds good, play it!