Blues Scale

The blues scale is the minor pentatonic with one note added — the flatted fifth, called the "blue note." That one extra note is responsible for the signature tension and expressiveness that defines blues phrasing. You can bend into it, slide out of it, and use it to connect the fourth and fifth in ways that sound mournful and vocal. It's one of the most expressive scales in Western music because it lives between the cracks of equal temperament.

Construction

Start with the minor pentatonic: A C D E G
Add the ♭5 (the blue note):    A C D E♭ E G

Formula: 1 — ♭3 — 4 — ♭5 — 5 — ♭7
Steps:   m3   W    H   H   m3   W

Blues Scale in All Common Keys

Key	Blues Scale Notes	Blue Note
A	A — C — D — E♭ — E — G	E♭ (D#)
E	E — G — A — B♭ — B — D	B♭ (A#)
G	G — B♭ — C — D♭ — D — F	D♭ (C#)
D	D — F — G — A♭ — A — C	A♭ (G#)
B♭	B♭ — D♭ — E♭ — E — F — A♭	E (E♮)
C	C — E♭ — F — G♭ — G — B♭	G♭ (F#)
F	F — A♭ — B♭ — B — C — E♭	B (B♮)

The Blue Note in Context

The ♭5 is usually a passing tone — not a place to sit and sustain. The classic moves:

Bend: push the 4th up toward the ♭5 and release
Slide: glide from ♭5 to 5 (the natural fifth)
Approach: use ♭5 to approach either the 4th or the 5th

Over I7 (e.g., A7 chord):
  Blue note is E♭ — a tritone against the A root
  It also clashes with C# (major 3rd of A7)
  Moving to E (natural 5th) resolves the tension
  This dissonance → release is the blues sound

Blues Scale vs. Pentatonics

Scale	Notes (in A)	Character	Best over
Major pentatonic	A B C# E F#	Bright, country, resolved	Major and dominant chords — I and IV in blues
Minor pentatonic	A C D E G	Dark, rock, confident	Minor chords, dominant 7ths, blues (most versatile)
Blues scale	A C D E♭ E G	Tense, expressive, vocal	12-bar blues, specifically for the I7 chord

Most blues players don't use any of these exclusively. B.B. King was a master of blending the minor 3rd (from minor pentatonic) with the major 3rd (from major pentatonic) over the same I7 chord — bending between them. That ambiguity between major and minor is the blues. The ♭5 is one more colour in that palette.

The 12-Bar Blues Context

12-bar in A: A7 | A7 | A7 | A7 |
              D7 | D7 | A7 | A7 |
              E7 | D7 | A7 | E7 |

A blues scale works best over the I7 (A7) chord.
Over IV7 (D7): shift your thinking to D blues scale, or use A major pentatonic.
Over V7 (E7): E blues scale phrase, or resolve strongly back to A.

Many guitarists stay on the A blues scale throughout — the ♭5 (E♭) creates
more tension over D7 and E7 than over A7, which can be used expressively.

Where the Sound Comes From

The blue notes in blues music — the ♭3, ♭5, and ♭7 — reflect the collision of African musical traditions with European harmonic structures. African music used microtonal inflections and pitch bends that don't map cleanly to the 12 equal-tempered semitones. When these vocal traditions met the fixed-pitch piano and guitar, the "bent" pitches became the flatted third, fifth, and seventh — notes that sit between the European scale degrees. The tension between a minor-flavoured melody and a major or dominant chord underneath isn't a mistake. It's the structural principle of blues harmony.

References

• Rawlins & Bahha — Jazzology
• Levine — The Jazz Theory Book
• musictheory.net — fundamentals ↗