Chromatic Tones or Something Else?

Q: It seems that in every book I read, everything comes down to scales. But often in some solos I analyze, I see notes which are not in the key center. For example:

The solo is in the key of A Major (the sheet tells me so as there are three sharps, F#, C# and G#).
But I find many times the notes f natural and G natural in the solo. They are not in the key of A major. It sounds ok but I get very confused. What is going on?

A: To really understand what is going on, more important than the key signature, is the chord in relation to the scale being played. Just because a song is written in the key of A major, doesn't mean it stays there the whole time. In your example of A major, the F and G natural notes can be many things. An A major scale with the G natural is a A mixolydian scale. Is the chord A7 while the G natural shows up? An A major scale with both these notes flatted is a D melodic minor scale. Is the chord a chord that works with this scale, like a C#7, G7 or Bmin7b5 chord? There are many possible explanations. 

If these notes fall between normal scale tones, they very well may be passing tones. Both these notes are common passing tones as they come between two scale tones. The G natural falls right between the 7th and 6th and the F natural, right between the 6th and 5th. Pretty standard fare for Jazz, especially if they are falling on the up beats.

-CJ