This is a more of an advanced theory lesson but the novice guitarist can grasp some of the material. In this video below I am guitar soloing to a metal backing track in the key of Eminor. When writing or improvising a guitar solo you need absolutely know two elements: the key you are in and the chords that you will solo over.
The backing track is in the key of Eminor and and chords that are being played is an E5 chord or (power chord) which is also combined with E pedaling palm mutes. Pedaling means you are continually played the same note. The chords after the E5 chord that are being played are called double stops. Double stops are when two notes are played harmonically (together). Double stops are not considered chords because their needs to be 3 notes minimum to be played harmonically to classify them as a chord.
The most usual double stops are made of different intervals played harmonically. After the E5 chord is played, the double stop is B and E. this is an interval of a 4th, followed by an interval of C and E. Which is a major third. After this sequence it is a G5—F#5/ G5. / abbreviation is a slide. Yes, power chords are a type of double stop but they consist of a root and 5th interval thus the name E5,G5,F#5 etc.
If we write out an Eminor diatonic scale, it would go as follow: E—F#—G—A—B—C—D. If we write out a pentatonic scale (meaning 5 noted) it is the same as the diatonic but with 2 less notes: the 2nd interval and the 6th. E—G—A—B—D.
Seeing the diatonic scale the E5 chord are the notes E,B,E, then the double stops B,E and C,E. Then G5 which is G,D,G and F#5 which is F#,C,F#. Knowing what the chords are, we can know identify that we can play safely an Eminor pentatonic scale and an Eminor diatonic scale and it would sound fine.
What I wanted to do in this solo was to use other scales outside of the diatonic and pentatonic. The scale I went with was a E Phrygian Dominant scale which comes from the A harmonic minor scale. If I write that scale out it would be: E—F—G#—A—B—C—D. If we compare this scale to the minor scale, we see a lowered scale degree 2 F# to F natural and G natural to G#. These two notes F natural and G# in this key make this scale exotic sounding, almost like a diminished or arabic.
The reason why I am able to play this scale over these chords is because the First chord is an E5 chord, EB. The Phrygian Dominant scale is safe to play over it because there are no notes in that E5 chord that I have altered and are in the scale. These are called chord tones.
The B,E double stop is also safe to play this scale over as with the C,E double stop. When a chord is given to solo over the lead guitarist is at the mercy of what chord. Those note in the chord you can not alter them or it will sound bad unless to play them quickly as a passing tone. Every other note outside of that chord is FAIR GAME. You can choose whatever notes you would like as long as it relates to the key in some way.
When the rhythm moves to the F#5 chord there is an issue presented. The E Phrygian dominant scale utilizes an F natural which will make it clash. As well as the G5 since the scale utilizes a G#. In this situation, you must not embellish those notes in your lead playing or the sound will clash. You need to either quickly pass through them or not play those notes altogether or make your way to a chord tone that is shared in the scale such as E,A,B,C,D. Alternatively you can embellish the note of that chord such as the F# or G natural. Ultimately the musical judgment should rest on your ears because sometimes there is a case to be made for either or options.
The bridge section of the solo introduces one new progression, an E5 to C5. The C5 consists of the notes C and G. Here is a situation where I sneakily use the G# to F natural by PASSING G# through G natural an F natural quickly to the note E. I once again do a final run using the E Phrygian Dominant scale and getting through the notes F natural and G# while the chord is still in E5 and end the scale safely on the tonic E when the C5 plays.
I find many of my students using a one scale fits all scenario. What I teach them to account for are what the chords are and what scales are available to use when they play. Most of them create chord progression that sound great but they don't know they have modulated (playing in a different key from your starting key) or they don't realize the chords they're using have been altered in some from and that one or two scales they were using didn't sound good over them.
I hope this free lesson was fun! Please, reach out to me for guitar lessons if you'd like to improve your guitar playing.