Watch the The I7 Chord online guitar lesson by Jeff McErlain from Focus On: British Blues

As a guitar player the first scale we usually learn is the minor pentatonic scale. Of course this is an awesome scale that we could spend a lifetime using and perfecting, but we can spice up that scale immediately by adding in the natural third. This is an essential sound in British blues.  If we look at an A7 chord—the I7 chord in an A blues—we see the notes are AC#EG. An Am pentatonic scale is spelled ACDEG, and an inherent "problem" appears that we can see clearly when we write out the scale. The chord has a C# in it and the scale contains a C natural. Those two notes can create quite some tension. This tension can be a good thing if we know how to use it properly, or a bad thing if we don't. The first thing we can do is add that C# into the a minor pentatonic scale.  Jam along with the included track using this new scale, paying close attention to the resolutions I show you in the examples.  The great thing here is if you were not already familiar with the sound, it is fairly easy to get under your fingers because it is so common.  I say common with my tongue in my cheek because it took me years to figure this out! :-)