Below is a paragraph that attempts to explain what happens when non-diatonic tones are introduced into a melody. I'm copying it word for word:
While the key signature does establish the rules we must use when playing a melody, strange things happen when we begin to improvise. Writers and arrangers frequently incorporate non-diatonic tones, accidentals (sharps-flats) in order to move a melody along in a desired direction. This in turn can create chords out side the original scale. This occurs because individual chords appear in more than one key. The use of accidentals also creates non-diatonic harmony which in turn requires non-diatonic chords to be used. The chords are created by adding a note not within the scale specified by the key signature.
I'm confused about what the sentence: "This occurs because individual chords appear in more than one key" has to do with the rest of the paragraph. I'm unable to see the connection between this sentence and the rest of the paragraph. I'm hoping someone can show me what I'm missing here. Anyone?