Dominant 7th chords
Among the diatonic chords of a key, the dominant 7th chord (built on the 5th degree of the key) is arguably the chord that has the most tension in it. This is because it contains the tritone (augmented 4th or diminished 5th interval) between the 3 and the b7 of the chord, and the tritone is an extremely dissonant interval.

But jazz cats are always looking for even more tension…
One way to add more tension to a dominant 7th chord is to stack up another third interval on top of the pile, resulting in a dominant 9th chord: (R, 3, 5, b7, 9).
For example: B9 = (B, D#, F#, A, C#).

That’s a great chord that will do wonders in blues and/or Western swing, for example. But it is still a relatively tame diatonic chord. For extreme tension, we need to add non-diatonic tones.
Enter the…

Altered dominant 7th chords
We are going to apply some (heavy) surgery to the dominant 7th chord, so let’s prepare it. We’ll start from a minimalist version, called a “shell voicing” consisting of the root, the 3rd and the b7: (R, 3, b7). For example, (B, D#, A). By the way, shell voicings are great for comping!

Now we dress up the shell voicing with any of the following four non-diatonic chord tones: b9, #9, b5, #5; we can add an altered 9th, or an altered 5th, or both.
This gives us the following possibilities:
– (R, b9, 3, b7), e.g. (B, C, D#, A) = B7(b9)
– (R, #9, 3, b7), e.g. (B, D, D#, A) = B7(#9)
– (R, 3, b5, b7), e.g. (B, D#, F, A) = B7(b5)
– (R, 3, #5, b7), e.g. (B, D#, G, A) = B7(#5)
– (R, b9, 3, b5, b7), e.g. (B, C, D#, F, A) = B7(b9,b5)
– (R, #9, 3, b5, b7), e.g. (B, D, D#, F, A) = B7(#9,b5)
– (R, b9, 3, #5, b7), e.g. (B, C, D#, G, A) = B7(b9,#5)
– (R, #9, 3, #5, b7), e.g. (B, D, D#, G, A) = B7(#9,#5)

The 7(#9) is sometimes called the “Hendrix chord” because Jimi Hendrix used it a lot (e.g. in Purple Haze); he’s credited for being the one who introduced this chord in mainstream rock, but of course he did not invent that chord.
This chord is interesting in a blues context because it contains the minor 3rd AND the major third. For example, E7(#9) = (E, G#, D, G) with a G and a G#.

All these chords are called “altered dominant chords”, and they are usually notated Xalt7; for example, all the chords above are Balt7 chords. When notated this way, you are always free to alter the 9th or the 5th or both, as you see fit – as long as the alteration doesn’t clash with the melody!
If the composer wants a specific alteration, he’ll mention it; for example B7(b9,#5).

Those altered chord tones provide great opportunities for smooth chromatic voice leading; for example: G7 -> G7(#5) -> C

The altered scale
Let us now take all the tones of that altered B7 chord and put them in a row:
(R, b9, #9, 3, b5, #5, b7) = (B, C, D, D#, F, G, A)
If we rename D# into Eb we get a proper scale: (B, C, D, Eb, F, G, A)
whose intervals w.r.t. the tonic are (T, b2, b3, b4, b5, b6, b7)
and which is called, aptly enough, the “altered scale”, since all its degrees are altered!
It is the scale to be used to improvise over an altered dominant chord.

Looking at the intervals of that scale, we notice that they look a lot like the intervals of the Locrian mode: (T, b2, b3, 4, b5, b6, b7); only the 4th degree is different.
For that reason, the altered scale is also called the “Super-Locrian scale”.

It just so happens that the altered scale (a.k.a. Super-Locrian) is also the 7th mode of a melodic minor scale!
The C melodic minor scale is (C, D, Eb, F, G, A, B); its 7th mode is (B, C, D, Eb, F, G, A) which is the altered scale.
This is one reason why jazz musicians are so fond of the melodic minor scale… But we don’t need to worry about this…