Jazz Improvisation Almanac, Outside Shore Music Online School" . http://www.outsideshore.com/school/music/almanac/html/Music_Theory/Jazz_Scales/Blues_And_Bebop_Scales/Blues_Scale.htm .   ^ Ewen, pg. 143 ^ Grace notes were common in the Baroque and Classical periods, but they acted as ornamentation rather than as part of the harmonic structure. For example, Wolfgang Amadeus Mozart 's Piano Concerto No. 21 has