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4. TENSIONS
Every scale related to a chord contains some notes not belonging to the chord. These notes, when played along with the chord, work as Tensions. As a general rule, these tensions correspond to the 9th, 11th and 13th of the chord. Depending on the tensions contained in the scale, and if altered or not, we get different sensations of sound.
In IMPROCHART, these tensions are indicated to the right of the scales. For example, if we go back to the Em7 chord (Fig. 3), we can see that the C M (Major) scale contains the -9 and -13 of the chord, that is, notes F and C, respectively. As nothing is said about the 11, it is understood that this scale contains the unaltered 11, that is, note A. In the same way, the B hm (harmonic minor) scale contains the +11, but the unaltered 9 and 13. The D M scale, on the other hand, contains the unaltered 9, 11 and 13.
This system used to indicate the tensions is also useful in the reverse sense, that is, when the tensions are indicated in the chord itself. For example, if the chord which we want to improvise on is Em7/+11, we will have to look for a scale related to Em7 which also contains the +11. So, a possible choice is B hm. This scale would also be useful for improvising over Em9, Em9/+11 or Em6/9 (the 6th is equivalent to the 13th).
In the cases of Bebop, Pentatonic and Blues scales, only those tensions which are particularly relevant are indicated.
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