Original: "Blue Note", The New Grove Dictionary of Jazz, Second Edition, London (/O2002/O) At its most basic, a single version of this "blues scale" is commonly used over all changes (or chords) in a twelve bar blues progression.
Simplified: The semitone differences for a blues scale are 3-2-1-1-3-2 for instance - C-Eb-F-F # - G-Bb-C