Altered scales are used predominantly by jazz musicians to produce interesting tensions when improvising over dominant seventh chords. Altered scales are the same as jazz minor scales played a semitone higher, so you only really have to learn one to play the other. For example, the A altered Scale uses the same notes as a Bb jazz minor scale. If you know the Bb jazz minor scale, you also know the A altered scale!
Altered Scale Guitar Diagrams
In the altered scale guitar diagrams below, the root notes are shown as white circles.
Altered Scale Theory
An altered scale contains the same notes as a jazz minor scale played a semitone higher, e.g.: an G altered scale contains the same notes as an A flat jazz minor scale.
When played over Dominant Seventh chords, altered scales provide the following altered notes that will create tension: b5, #5, b9, #9
Notes in an altered scale
The notes in a G altered scale are: G, Ab, Bb, Cb, Db, Eb, F, G
The A flat and B flat are respectively the flat nine and sharp nine tones, the C flat and D flat notes are respectively the flat and sharp five notes.
Hear an example guitar solo using the altered scale, and play your own over a backing track here: How To Use Altered Scales.