When harmonizing a melody we often come across notes that pass between two chord tones in a melodic line. We know that we can harmonize the chord tones easily enough with whatever the indicated harmony happens to be, but what about those notes in between? If we decide we want to harmonize these passing tones the easiest and most effective way is through the use of the diminished seventh chord. For example let’s use the A minor scale and say our melody is the ascending form of this scale (a b c d e f g a) and the indicated harmony is the A minor seventh chord. Notice how the first, third, fifth and seventh tones of the scale are members of the Am7 chord (a c e g). The notes in between are the passing tones. We will make a slight adjustment to the scale by including the leading tone G-sharp. This gives us a nice eight tone scale which works perfectly for this example. The new scale is now (a b c d e f g g# a). Now you can see that the passing tones spell the G-sharp diminished seventh chord (b d f g#). Since this particular chord has a symmetrical structure consisting of consecutive minor thirds we can name the chord using any of the four tones and easily invert it by moving the same voicing up the fingerboard at intervals of three frets or minor thirds. I chose to call it G#dim.7 to better indicate its leading tone or dominant function. So let’s try this for practice. Start with the A on the first string fifth fret and harmonize it with any voicing you like of an Am7 chord. Next, move to the B at the seventh fret first string and harmonize it with a G-sharp diminished seventh chord. Continue this pattern of Am7 to G# dim.7 through the complete eight tone scale (see examples below) and you will hear a familiar sound found in many jazz chord/melody arrangements. Harmonically, the G#dim.7 simply functions as a dominant to the tonic Am7, so in essence we are just moving from I to V and back again. The concept works equally well in major keys as long as you remember to use the eight tone scale as shown above derived from the relative minor key. For example in A major the scale would be (a b c# d e f f# g# a). The tonic chord will be AMaj7/6 and the passing chord will continue to be G#dim.7. Understanding this concept will give your arrangements a more authentic jazz sound. So the next time you come across a passing tone in a melody line try harmonizing it with a diminished seventh chord, you will be glad you did.