The blue fourth has acquired lots of alias's over the years. Known also as the "tritone", or perhaps as the sharp four or flatted fifth, the blue fourth is one serious player. The latter two names ( # 4 / b 5 ) are based upon it's intervalic relationship within various scales and chords. Personally, I think of the blue fourth as the tritone, to keep things simple. It's an easy word for me to remember and is used in other important aspects of the theory. This is in part due to how I understand and structure the theoretical principles of American music.
Although I digress from the topic at hand, my version of the theory is based upon the premise that the tritone pitch was borrowed from the more urban, European musical system of equal temper and simply added to the more rural, minor pentatonic group of pitches, whose color is so wonderfully represented in the music of rural or indigenous peoples globally even today. For the early African Americans, the people who created the blues, had a real need for such a pitch. For the tritone is among the darkest, most discordant of the equal tempered intervals, which when added to the minor pentatonic color creates the blues scale, the well from which much cool American music is drawn.
Lets reshape a two bar minor pentatonic phrase into a two bar blues lick by simply adding the tritone within the lick. The difference is subtle but the color change is distinct. At least, I think it is. Dig the rhythmic displacement of this lick, drives the groove nicely. Example 1.
Can you hear the color change in the above example? If not, try again, it's there, but subtle, yes? Is being subtle important? Well maybe, depends on one's artistic direction of course. The cool thing here is the idea that the presence of the tritone in the line shades things in a more bluesy way, from which the idea emerges of the potential for the tritone color to almost instantaneously invoke a blues potential. The music does not have to totally go there, but the improvising artist can hint at their blues roots by deftly inserting a bit of the #4 color in their musical musings. Splitting theoretical and stylistic hairs? Of course, but this tinting the musical colors with the tritone happens a lot and is a part of how one thinks as they create their lines, to tell their story, allowing "proppa blues testimony to occur. "Cruise to the tritone page for a more in depth look at this wonderful color.
There are lots of cool cliché blues ideas that rely on the tritone for it's added spice and character and darker color. Here is a common blues vamp emphasizing the tritone. Example 2.
Personally, this above lick is totally jungle. So many images come to mind. We can perhaps begin to see it's power in shaping and effecting the elements that surround it. In blues playing, the tritone is almost "stand alone" within the blue notes, further on we'll use the tritone in other configurations where its role is more as a team player in creating dramatic, non diatonic, hybrid, musical colors. Here is the "flip side" to the above lick. Example 3.
Oftentimes this kind of dissonant line is balanced with a blues lick into a four bar phrase known as a vamp. Example 4.
Using the blue fourth in the minor tonality creates a very sad, plaintive voice. Example 5.
Here is the blue fourth in an old cliché idea. This lick has been used everywhere, by kids on the playground, television commercials, marching bands sounding out the impending doom of their opponents. The tonic / minor third motion sets up the tritone splash. Example 6.
Sound familiar? No wonder the older Euro theorists poo poo'd this color.
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"Make everything as simple as possible, but not simpler." Albert Einstein