The 12 Bar Blues is a chord progression not only used in blues but in all types of jazz.
The blues is a very popular American music form used in the 20th century and is still being using in the 21st century.
This progression includes a harmonic structure of twelve measures with each measure having 4 beats. This chord pattern sequence, written in Roman numerals, is repeated throughout a jazz piece of music. Rather than using the dominant 7th chords on I and IV chords, use the Maj7th chords on the first and fourth degrees of the scale in the key you are in.
Let's apply this format to the key of C Major.
It can be applied to any key signature.
~The I chord (triad) in the major key (scale) of C is the C chord
(C, E, G). In jazz the 7th note above the root is added, thus CMaj7 (IMaj7):
C E G B.
The root, or the bottom note of the chord that names the chord, is "C". The Roman numeral "I" names the function of that chord, based on the first note of the scale you are in. In this example you are in the key of "C".
~The IV chord in the Key of C (starts on the 4th note of the scale of C) is the F chord (F,A,C) with its root as the 4th note of the scale of C. Adding the 7th above the F in the key of C, the note E, the chord becomes IVmaj7 in the key of C: F A C E
~The V chord in the key of C (starts on the 5th note of the scale of C) is the G chord (G,B,D) with G as the root.
~The 7th note above the root of the G chord in the scale of C is F which makes the V7 (also called the dominant chord in the key of C), G B D F.
~Let's move this pattern to the key of D (remember the I chord triad in the key of D is built on D with the added 7th note, D F# A C#). The key of D has an F# and C#. Therefore there is an F# and C# in the IMaj7 chord. The IVMaj7 chord (starts on the 4th note of the scale of D) is G B D F# and the V7 chord (starts on the 5th note of the scale of D) is A,C#,E,G. (Remember C# is in the key of D.) The 5th note of the scale of D is A. Try playing these chords: IMaj7, IVMaj7, V7 using the 12 Bar Blues progression in the key of C, then in the key of D. Can you feel the V7 chord (A,C#, E, G) pulling you toward the I chord which is D, F#, A, C# by way of the IVMaj7 (subdominant chord) (G,B,D F#)?
The V7, IVMaj7, IMaj7 progression pattern is used in jazz.
~Now it is your turn to play the I, IV, and V7 chords in the key of C and then in the keys of G and D. Then try the chords in the keys A using the A, D and E7 chords. Use them the same way as above in succession to form your sequence of harmonic changes (going from one chord to another) in the 12 Bar Blues pattern. Let your ears be your guide while playing and writing harmony.
~In the process of writing the harmony for your song, or improvising on these chords by using scale passages and arpeggiated triads, remember that an arpeggiated 7th chord is a 4-note chord (such as C E G B) played one note at a time.
~When you are satisfied with your sound, notate the chords on music staff paper or improvise chord patterns on the notes that go with your melody and words.
~Experiment on your piano with this pattern of chords in learning how to write a song.
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