Watch the Scale Applications 2 online guitar lesson by Robben Ford from Blues Revolution
Building on the previous tritone lesson, Robben demonstrates how to view a C#7 as the tritone substitution for a G13 chord (like a G13 with C# in the bass: C#-F(E#)-B-E-G). We can use the diminished scale over this chord: G-Ab-Bb-B-C#-D-E-F-G.
In the blues, the tritone substitution usually shows up on the two beats before moving to the IV chord (for example, to C in a G blues). Robben likes to use chromatic tones (notes a semi-tone below the target note) and these are the same types of motion as the C#-C root movement that we previously observed.
As an added bonus, Robben demonstrates a great diminished pattern made famous by sax great John Coltrane, where you play a note, then go down a whole step and up a perfect fourth in sequence. For example, G-F-Bb-Ab-Db-B-E-D-G.