Watch the Jam Track #1 online guitar lesson by Brad Carlton from Guitar Lab: Latin Rock Lead
We will be dealing with a four bar progression which can be seen in chart number one. Notice that measures one and three are full measure of the prevailing chord and measures two and four are what I term "two and two" which means you have two beats on each chord. Knowing the progression is critical in order to be able to anticipate the chord changes. There are two ways you can view this progression in terms of theoretical analysis. The theoretical key for this progression would be G major because the Em to B7 chord progression is a vi III7 in the key of G. This means that during the Em chord you would play the E Aeolian mode which is a G scale played from E to E. The B7 is non-diatonic which means that it's not in the key of G major. The note that changes is that the D# which is the third of the B7. This explains the III7 labeling because the three chord should be a minor to remain diatonic to the major scale. The scale you'll be using over this B7 is B Phrygian dominant which can be viewed as mode five of E harmonic minor. A quick easy way to get this scale would be to simply take the G major scale and change the D note to a D#. The street key analysis of this progression would be i to V7 in E minor.