When it comes to theory and it's application in solos. Harmony will usually keep to the structure of music theory, while melody (solo) will often play against it. This means that the harmony will stick with key signatures and chords within a key. The solo however will use scales that are often contrary to the key.

Example of what I mean. Chord progression as I said before would be in C major. While the scale you might use for the solo is the C minor pentatonic. This is common practice in rock and blues. The third of C minor (Eb) is the blues major tone (b3) of the C major blues scale.

It's not preferrable to the ear to play a solo using the C major scale over the key of C major. Although the notes all sound right, this effect is rather dull and monotonous.

As you can see, if you superimpose any scale over a chord progression in a key. You will be left with all 12 notes being used. Some scales work really well, and some don't. The most compatiable can be found by:

Looking at the triad of the key your in. Say A minor, the notes are A, C, and E. A scale with those three notes in it is compatiable. Even more compatiable is if the key note and the root of the scale are the same. This second statement is broken by jazz players who will play scales with different roots then the key of the song. This is rather difficult for a beginner, stay with the same root to same key until you better understand melody/harmony.

If your follow the above, here are some alt. examples of scales over keys.

C major: C lydian, C mixolydian, C spanish phyrgian, etc.
A minor: A minor blues, A melodic minor, A phyrgian, etc.

That should at least point you in the right direction, rather than just saying any notes will work.

[Edited by noticingthemistake on 02-15-2004 at 10:48 AM]