Many rock songs, soul tunes, funk classics, and anything that is at all blues based, which is almost all of the music we hear today makes use of certain melodic and harmonic devices that are used so regularly that they’re considered clichés, which isn’t necessarily considered bad. A collection of guitarists as diverse as Steve Vai, Joe Walsh, Jimmy Page, Jimi Hendrix, Joe Satriani, Steve Cropper, Angus Young, Buddy Guy, Slash, Tony Iommi, etc., have all been known to use blues clichés, whether in a blues context or otherwise.
One such cliché might be entitled “sliding 6th chords”, or an interesting chord-on-chord approach. Jazz players like to use chord substitutions, or seemingly unrelated chords in place of standard chords. In this blues cliché, we will use what could be considered a complicated jazz concept that can be used quite simply with great sonic results. Most seemingly complicated concepts can be explained and used in a simple fashion. Music theory comes after the music. The sound is first and the labels, sometimes more complicated than is necessary, are thought up after the fact to justify and explain the sound.
Sliding 6th chords are utilized when we substitute a minor triad for a related major triad. Let’s assume that we are playing over a C7 chord. This might be in a rock song, a country song, the blues, even as far afield as Brazilian forros. All of these and other styles may use a dominant 7th chord as the tonal center, or the “I” chord. The C7 chord would be built with the notes C, E, G, and Bb, the root, major 3rd, 5th, and lowered 7th notes of the C Major scale.
If the related key signature allows it, we can usually include the unaltered 9th note and the unaltered 13th notes. In this case, the 9th would be the D note, and the 13th would be A note.
Notice that D can also be numbered 2 in the C Major scale, and the A could be also be numbered 6. We number them 9 and 13 to define them as being part of a 7th chord, as opposed to being added to a triad. Chords are built in thirds, so the 13th chord would be spelled C, E, G, Bb, D, F, A, or root, 3rd, 5th, 7th, 9th, 11th, and 13th. The 11 doesn’t work well with the minor tonality, masking the important 3rd of the chord, so we tend to leave that one out.
If we’re playing over a C7 chord, or even a CMaj triad that fits the C7 function, such as one that resolves to some sort of F chord, then we may be able to use Bb, D or A as consonant-sounding melody notes over this chord. If we take the root and the 3rd of the chord, C and E, and add the A note, the 6th, in place of the regular 5th, G, then we have what could be considered a functional, incomplete C6 chord (with no 5th note), or more easily identified as an Amin triad.
Another minor triad that works well in this context would be the Gmin triad. It is built with the G, Bb, and D notes. Notice that these notes are all in the C13 chord, but since there is no A note in this chord, more correct to consider this collection of notes to be the upper structure of the C9 chord.
The most common usage of this information is to slide from this incomplete 6th chord (C, E, A), to the incomplete 9th chord (Bb, D, G). When the ear hears this triad slide, the notes of both triads are considered over the initial chord (C or C7), suggesting a C13 chord. The most common guitar fingerings are seen as the related minor triad with it’s root on the top of he voicing. Most often, these are played on the first, second, and third strings, or on the second, third and fourth strings.