what is a key? part v -- caveat ii

Today's post will be concerned with another caveat when considering keys. This one could also be called About Non-Chord Tones.

CAVEAT NUMBER TWO
So far in our series we have encountered a situation where we had to exclude some tones from a key because they were part of another key (see this post). Here we're going to look at a similar situation, but we are going to exclude notes that are not part of another key. Consider Scott Joplin's The Entertainer:

All the notes highlighted in red are not part of the C major scale. This should be somewhat obvious, because C major contains no notes which are sharps or flats, and all of the highlighted notes are sharps and flats. But not only are they not not part of C major, they don't form part of a chord which is in another key, either. Consequently these sorts of notes are called (very creatively) non-chord tones. 

There are actually a fairly large slew of non-chord tones, but technically classifying them all isn't really important for our purposes here. The main thing to keep in mind is that all twelve tones of the chromatic scale can be in play without destabilizing the tonal center. This is good to know when improvising or writing melodies. 

There is a lonely note above highlighted in blue. Upon inspection can you see why it has been singled out as a different kind of chromatic note? 

It's because that Bb is part of the chord, the 7th of the C7 chord, which is the V7 of the key of F major, and low and behold F major is the next chord.

Endnote "Caveat to a caveat"

In the Joplin tune the lonely D-sharps at the very end of the two lines could be considered as the #5 of a G augmented triad. In which case the D-sharp is in fact a part of a chord, but the chord it is a part of actually reinforces C major, and not another key area. More on this in a later post!