What does it mean to be 'out of key' in music?

What does the term "key" mean in music theory?

• Answer:

  The key of music -- referred to by other names as well, such as tonality or tonal center -- describes, in layman's terms, the chord that we perceive to be "home base" in that music. This means for example that if a passage or song is in the key of C major, we will probably not feel complete closure unless it ends with a C major chord, and our ears will expect it to return there at some point. Much of the harmonic interest of Western music has traditionally been generated by seeing how far we can move away from a tonal home base, and in what ways we can do so, while still being able to successfully and sensibly get back to it eventually. When music changes keys while in progress, whether temporarily or permanently, this is usually called modulation. Importantly, changing the key of a selection up or down (whether I'm going to sing this song in C minor or A minor, say) changes the sound of the music (pitches it higher or lower) but does not in and of itself fundamentally alter any of the musical content of that selection. That is, you will recognize the song "Happy Birthday" whether it is in F major, or D major, or C-flat major. It will be easier for you to sing that song in some keys than in others because of your particular vocal range, but it will remain the "Happy Birthday" you know and love (or loathe) in whatever key we place it.