The Meaning of the Commonplace

I was at work thinking about the words commonplace and common place; so I looked to the dictionary. My question was "was there a difference, if any, when the one word became too?" Well not really the definition for commonplace was basically: lacking originality. Ok it fits. But further in the definition came commonplace book. It means: a journal in which one records significant observations, poetry, etc.
Think about it.