Why do some songs not fit into any one key?

Anything goes in the world of music composition. Many artists play solely by ear by piecing ideas together through trial and error. But there are fundamental concepts that underlie all music like scales, chords, progressions, modes, etc.

If a song doesn't all fit into one key, then it's a combination of different keys. Sometimes this is thought of as borrowing elements from other keys. In this case, you have to break the song down and separate it into pieces that each fit into a key.

Another possibility in this type of situation is the usage of scales beyond the pentatonic and major scale patterns I teach. For example, the harmonic minor and melodic minor scale. But you need to understand pentatonic and major scale theory first as advanced concepts stem from them.

Going through this trouble to understand a song's construction is not usually necessary in styles that don't require much melody or harmony. For example, some heavy metal riffs are simply based on chromatic movement or rhythmic noise. It can be kinda pointless to try and connect these things to some sort of structured guitar theory.

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Mr. Desi Serna
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What is the "Key" of a song?

"Key" can refer to a several things. They include:

1. The key signature or parent major scale
2. The root
3. The first note or chord of a song.
4. The mode

In most cases when a guitar player says "key," he is referring to the chord or note that sounds like the tonal center of the song. This root may not always match the actual key which should be based on the parent major scale. For example, "Gloria" by Van Morrison revolves around, and resolves on E, but the chords are derived from the A major scale's V IV and I (this produces E mixolydian mode).

Sometimes guitar players will name a key simply by whatever note or chord is first. For example, "Sweet Home Alabama" by Lynyrd Skynyrd starts on D, but G is the strongest tonal center, and the chords are derived from the G major scale's V IV and I. The majority of the melodies, riffs and solos are based on G major pentatonic and G major scale. It's more correct to say the song is in G, but most players call it D.

As you can see, guitar players have different meanings for the word "key," and musicians can examine a song's structure from different perspectives. A good approach to learning guitar theory is to always start with the parent major scale, then identify which note is functioning as the root.

Play Until Yer Fingers Bleed!
Mr. Desi Serna (Google me!)
http://www.Guitar-Music-Theory.com
Scales, Chords, Progressions, and More

Guitar Theory Includes Scales, Chords, Progressions and More.

Guitar Chords
Consider your favorite song. It probably has guitar chords in it. Where do the chords come from? How are they built? What is the chord progression, that is, what are all the chords used, how are the chords combined and in what order? Why can the chords be combined in this manner?

Guitar Scales
Does your favorite song include a melody, guitar riff, lead guitar solo or bass line? Parts like these are derived from guitar scales. How are scales made? How are scales played on the fretboard? Which guitar scale does your song use? Why does the scale work over the chords and chord progression?

Guitar Keys and Chord Progressions
Do you think these questions are relevant? If you had to learn a song would it help you to understand how it was put together? How would knowing a song's key, chord progression and scales used effect how quickly you learned it? Would having a complete grasp of a song's inter-workings help you remember it better? Have you ever tried composing your own guitar music? When you know exactly how scales, chords and progressions are combined to make music, what will you do with this newfound knowledge?

Guitar Theory
As you can see, there are many benefits to learning the details of music and how they specifically apply to the guitar fretboard. Aside from being interesting, guitar theory can really open up your playing and greatly impact your rate of development. You'll pick up on new songs faster, retain more of what you learn, jam, improvise and compose your own music.