Altered Scales & Tritone Substitutions

Intro

In a simple chord sequence, in which all the chords come from the same key, it’s easy to select the scale to improvise over the sequence. The standard choice is to use the scale of the key.

In this example (a I-VI-II-V7 in G), in which all the chords are in the key of G major, the simplest choice is to use the scale of G major to improvise over the entire sequence.

The chords are said to be diatonic to G major, because every note in each chord is in the G major scale.

Extensions

You can easily extend the chords by adding more diatonic notes such as 9ths, 11ths and 13ths:

Adding these extensions to the chords creates a richer texture with more colour but it doesn’t affect the scale choice, since we’re still only using notes of the G major scale to create chords with.

In jazz, adding altered extensions is a skilled operation: altered extensions can fundamentally change the sound of the chord; they can change the role of the chord (known as the function); and, of course, altered extensions can change the choice of scale to play over the affected chord.

However, there’s one chord type where altered extensions can actually emphasise the function of the chord and create a more interesting sound: chord V7. Looking at D7, you can strip it back to its bare essentials by having a D (the root of the chord) and also F♯ and C (the 3^rd and the 7^th, which together create a tritone, which is the source of the tension in D7). No other note is necessary. The 5^th of the chord (A) doesn’t add any function or colour to the chord, so it really only needs three notes: the root, and the tritone pair above the root: D, F♯ and C.

V7(♭5)

One very common alteration is to flatten the 5^th. Here’s the same sequence again. D7 is now played as D7(♭5). The chord contains D, F♯, A♭ and C.

This 7(♭5) chord is pretty interesting. The notes it contains are also found in A♭7(♭5), an identical chord that is half an octave away.

Tritone Substitution

Try playing the sequence again, but this time substituting D7(♭5) for A♭7(♭5). You have the same notes, but you’ll notice that the root—A♭—descends chromatically from chord II and eventually on to chord I, creating a smooth, elegant sound. Since it is half an octave away, using this chord is known as a tritone substitution. Remember, D7(♭5) and A♭7(♭5) are essentially the same chord, since they contain the same notes. Which one you play will depend on the bass line you want to create.

Try these shapes—all of which can be played six frets higher or lower to ‘flip’ them from D to A♭ and vice versa.

As a guitarist or pianist, providing chords for an accompaniment, feel free to use the tritone substitution for V7 chords. Whether it sounds any good will be a question of taste and experience. You obviously need to let the bassist know, and any other musicians who might be playing chord notes. You’ll find tritone substitutions in bossa nova, swing, ballads, bebop… and it’s an essential part of a jazz musician’s repertoire.

Now try playing the D7(♭5) & A♭7(♭5) chords over this backing track. Each chord lasts for two bars, so the whole sequence is 8 bars long. In bars 7 & 8, try playing D7(♭5) or A♭7(♭5). They’ll both work fine. The backing track is 64 bars long, meaning the 8-bar sequence is played a total of 8 times through. I’ve created the backing track with the bass playing roots of D7(♭5) and A♭7(♭5) in bars 7 & 8. It’s ambiguous, to allow you to hear how interchangeable the two chords are.

 

Scale Choice for the Tritone Substitution

So… what scale do you use to play over it? Well, the ‘rule’ is this: any scale that contains the notes of the chord you want to play over is a candidate. It might not be obvious which scale contains the notes D, F♯, A♭ and C, but if you call the A♭ a G♯ instead, then there’s common scale that is an obvious choice:

A melodic minor contains all the notes in D7(♭5)—or A♭7(♭5), of course, since it’s really the same thing.

Playing it over D7(♭5) means we’re using the fourth mode of A melodic minor. You can see that the G♯ is a sharpened fourth, and that the original fifth of D7 (A) is actually still in the scale. Theorists will refer to this sharpened fourth as a ♯11.

[D7(♭5) could be called  D7(♯11) instead. In fact, since we’re talking about G♯ now instead of A♭, maybe #11 is a better description than ♭5…]

Anyway, the scale has a ♯11 (G#), and a flattened seventh (C), so the common name for this scale is lydian dominant.

The lydian dominant is very popular, especially over this kind of chord, but you could also play it over an unaltered V7 (because really we haven’t changed the fifth at all—we’ve actually just added a ♯11 so the scale doesn’t conflict with a standard V7).

Now try improvising over the backing track. Use a G major scale for the first three chords but, when we get to bars 7 & 8, try playing D lydian dominant: that is, A melodic minor. In fact all you’re doing is replacing G with G♯ for those two bars.

You’ll hear that there’s subtle additional tension in the V7 chord and your melodic ideas will control and negotiate that tension to create structure in your improvisation.

So, we now know that we can play D lydian dominant over D7(♭5)—and of course we can play it over A♭7(♭5) too.

Altered Scale

To finish off, let’s revisit our D7 chord. So far, we’ve added one altered extension, the ♭5 (or ♯11 if you prefer). Let’s see what happens when we add every possible altered extension. Remember, we only ‘need’ D, F♯ and C to make this chord D7. The flattened ninth, Eb, can be added, along with the sharpened ninth, F. We already have a ♯11, and the only other altered extension we can add is the flattened thirteenth. Altogether, then, we’d have D7(♭9/♯9/♯11/♭13), which looks a bit ridiculous. But actually all that’s happened is that we’ve just piled up as much tension as possible on top of the bare bones of a D7 chord.

Starting on D, let’s lay all these notes out in order—three chord notes and four altered extensions.

Spelling the same notes slightly differently makes it easier to comprehend:

It creates E♭ melodic minor, starting on the seventh degree. To come at this a different way, consider that E♭ melodic minor has the same relationship to A♭7(♭5) that A melodic minor has to D7(♭5).

The seventh mode of the melodic minor is also known as the altered scale, or the superlocrian scale.

Another way to produce the same scale is to take an ordinary major scale and raise the root by a semitone. This might be a useful way of visualising the scale. In the case of the D altered scale, you could create it by starting with a D♭ major scale and raising the root to D.

Try improvising over the backing track again. This time, over bars 7 & 8, try either the D lydian dominant or the D altered scale. The lydian dominant has a bright, fresh quality that just gives the V7 chord a little lift; whereas the altered scale creates a darker sound with more tension. Listen out for the ♭9 and ♯9 notes (E♭ and F in this key) and the ♭13 (B♭) for more interesting melodic possibilities.

Here’s my demonstration, soloing over the backing track. I’ve incorporated a few ‘features’ for you to hear. For the first four play-throughs of the sequence I’m using the lydian dominant scale, and on the other four I’m using the altered scale. See below for a few ‘notes’, especially over the V7(♭5) chord.

2nd time through [0:35]
The melodic minor scale contains a long stretch of whole tones—in this key, from C to G♯, and I’m using this part of the scale to create an expansive feel.

3rd time through [0:52]
I’m playing notes of an E chord—E, G♯ and B. They’re all notes of the A melodic minor scale, so even though I’m not playing over an E chord, these notes are a legitimate choice. Try other arpeggios that are contained within the A melodic minor scale, such as Bm7 and Caug, for a more unusual angle.

5th time through [1:27]
I’m playing notes of the altered scale in an ascending phrase, and I’ve timed it so that the final two notes are F (♯9) and F♯ (major 3rd), leading up chromatically to G, the key note, for the start of the chord I that follows.

6th time through [1:40]
Over the Am7 that precedes D7(♭5) I consciously played a G pentatonic phrase. Since the D altered scale (E♭ melodic minor) contains the notes of A♭ pentatonic, I was able to repeat the G pentatonic phrase a semitone higher over D7(♭5).

 

Footnote:

By the way, how do you write the chord symbol that demands an altered scale be played over it? In fact, the simplest way is to write D7alt. But any dominant chord with an altered extension, such as D7♭13, D7♯9, D7♭9♯11, D7♯9♭13 etc will be an opportunity to stretch out with an altered scale. Even a plain old D7(♭9), where you might normally play G harmonic minor, or a diminished scale, can stand to have an altered scale played over it. It depends how ‘in’ or ‘out’ you want to be—and on stylistic choices too. Experiment and learn to calibrate your sensibilities.