Blues
Blues originated in African American slaves in the southern United States.
Contents
Blue Notes
One of the main impacts of the blues was its incorporation of "blue" notes, which are actually inflections of particular notes played at a lower pitch than they normally appear in a scale. These notes could be bent or slid upwards to approach the note they replace.
Equal Temperament Theory
One theory behind the success of blue notes is based on the equal temperament tuning system. In theory, the 12 tones of the chromatic scale perfectly represent the most harmonic intervals possible when played against a common note, and these notes are evenly spaced apart in terms of frequency. In actuality, the most harmonic intervals are not evenly spaced. Equal temperament basically ignores this discrepancy, evenly spacing the frets on the guitar. The advantage of equal temperament is that the instrument can be played in any root note, and consonance of any intervals played against that root note is the same as the same interval played against a different root note. The disadvantage is that the intervals are slightly off from their more harmonious counterparts.
Just intonation would space the frets unevenly. This would ensure that for one root note of the 12-tone scale, all the intervals would be the most harmonic possible. The downside is that all other 11 root notes would not, and some would be noticeably dissonant.
Equal temperament is generally preferred because it minimizes the possible dissonance. But it still means that all the intervals are a bit off.
Blue notes attempt to reconcile this discrepancy. The major scale's major 3rd and major 7th are the most obvious candidates for blue notes: their equal temperament pitches are quite higher than their just intonation counter-parts. Using the minor 3rd and minor 7th in their place and bending upwards is able to hit the just intonation pitches.
Alternative Theory
The other common blue note is the flatted 5th, and this does not make as much sense in terms of the theory above. The equal temperament 5th is actually very slightly below the just intonation fifth. Thus, there is no need to use the flatted 5th to get "below" the just intonation 5th; and because the just and equal temperament notes are so close already, there is no need for a blue note. And to add complexity to the matter, the equal temperament diminished 5th is actually quite higher than the just intonation flatted 5th.
Furthermore, the blue notes can only be bent upwards towards their just intonation counterparts on instruments that allow notes to be bent. For piano and many other instruments, this is not possible.
My theory is that the blue notes are not supposed to get the player closer to the just intonation intervals, but to actually purposefully avoid them. The blue notes themselves are always further from the just intonation interval they are supposed to reach than the equal temperament interval. In the case of bending towards the just interval, rarely if ever will the note be bent quickly to that interval and sustained there. Rather, the blue notes are used more expressively, moving slowly towards the just interval and moving to the next note or simply ending as soon as or right before reaching it. In this sense, the blue notes represent a clearly "incorrect" note that is striving to get to the correct pitch, similar to the leading tone's invisible push towards the tonic note. In short, it is very similar to the effect of bending, where the value lies in moving towards rather than reaching the target.
Blues Scale
The common minor blues scale is the same as the minor pentatonic scale, plus the d5/a4/tt between the 4th and 5th. It is seen as the D# in the A blues scale below.
E--------------------------------------5--8-- B--------------------------------5--8-------- G-----------------------5--7--8-------------- D-----------------5--7----------------------- A--------5--6--7----------------------------- E--5--8-------------------------------------- A C D D# E G A C D D# E G A C
Notice from our blue note analysis above, the scale uses the m3 and m7, and incorporates the d5 - these are the common blue notes. Since the blue notes replace the M3, P5, and M7 of the major scale, the blues scale can actually be played over a major or minor key with the same root, although it will clash with the major scale if the blue notes aren't expressed as striving towards their major counterparts.
Blues Progression
The most common blues progression is the 12-bar blues, which consists of 12 measures, each featuring 1 chord per measure. Typically, the first four bars establish the I chord as the tonic. The next two bars introduce the IV chord before returning to the I chord for 2 more measures. Then we either get two bars of V or 1 bar of V followed by a bar of IV, before returning back to I for the last 2 measures.
1 2 3 4 5 6 7 8 9 10 11 12 I - I - I - I - IV - IV - I - I - V - V - I - I I - I - I - I - IV - IV - I - I - V - IV - I - I I - I - I - I - IV - IV - I - I - V - IV - I - V I - I - I - I7 - IV - IV7 - I - I7 - V - IV - I - V7
The last 4 bars are often called the turnaround, involving the V - IV - I or V - IV - I - V progression, which leads back into the beginning of the entire 12 bar progression. The V - IV - I progression is called the Blues Turnaround.
Blues Shuffle
Rather than simply playing sustained chords or strumming patterns, blues would often use a shuffle to indicate the harmony. This consists of alternating between P5, M6, and m7 intervals, almost like a riff, often employing some sort or swing feel.
x4 x2 x2
D-----------------|2-2-4-2-5-2-4-2-|----------------|
A-2-2-4-2-5-2-4-2-|0-0-0-0-0-0-0-0-|2-2-4-2-5-2-4-2-|
E-0-0-0-0-0-0-0-0-|----------------|0-0-0-0-0-0-0-2-|
x2
D-4-4-6-4-7-4-6-4-|2-2-4-2-5-2-4-2-|----------------|
A-2-2-2-2-2-2-2-2-|0-0-0-0-0-0-0-0-|2-2-4-2-5-2-4-2-|
E-----------------|----------------|0-0-0-0-0-0-0-0-|
Other simpler versions using just a bassline deliver the same meaning:
E-0--4h5----3--0----0--0----0--0----0--0-|