The blues originated in African American culture after the Civil War. Blues guitar is heavily influenced by vocal melodies that came from gospel and folk music, but the blues was first played on instruments that were more primitive than guitars. These instruments and influences dictated simple scales and progressions, making the blues an art of form and repetition.
Like classical music, the blues guitar scale is built from notes within a specific key. The first note of the scale is the same as the key, and is often called the “root note,” or “tonic.” For example, in the key of E, the root note of the blues scale is E.
The minor pentatonic scale is the foundation for most blues and rock melodies. “Minor” means that the scale's third, sixth and seventh notes are a half-step flat compared to the major scale, and “pentatonic” means that there are only five notes in the scale. For example, a C-major scale includes the notes C, D, E, F, G, A and B. A C-minor scale, with its flat third and sixth notes, is C, D, Eb, F, G, Ab and Bb. Finally, the minor pentatonic omits the second and sixth notes, leaving C, Eb, F, G, B.
The major pentatonic is the second most popular scale used in blues. Like the minor pentatonic, it consists of only five notes, but it is derived from the major scale. The fourth and seventh notes are subtracted from the major scale, leaving C, D, E, G and A in the key of C.
While they comprise the foundation of the blues, the major and minor pentatonic scales are present in all styles of music. Blues musicians set their music apart with the use of “blue notes” that technically don't belong in the scale. Any out-of-place note is considered blue, but when guitarists refer to the “blues scale,” they usually mean a minor pentatonic with an added, flattened fifth. For example, a C-minor pentatonic scale with a blue note is C, Eb, F, G, Bb, B.