Watch the I iv/I online guitar lesson by Brad Carlton from Guitar Lab: Slash Chord Science
This lesson presents the following bichordal slash chord progression: I iv/I. The street key for this vamp would be C. In this progression, each chord is coming from a different scale. To determine the theoretical key, consult appendix B which is a chart of the major scales and the diatonic chords are built from those scales. If we analyze this progression, the C chord could be a I in the key of C, a IV in the key of G, or a V in the key of F. The Fm chord could function as a ii in the key of Eb or as the i chord from the F melodic minor scale.
In the video I will demonstrate playing the complete slash chord (triad over the bass note) as well as playing along with the jam track in which the bass note is already being played in the rhythm section. You will see how I use different triad voicings to create a keyboard style of playing.
Diagrams 17a and 17b show these available triads over the entire fingerboard based on the three one note per string arpeggio forms that you learned in lessons one and two.