Diatonic decoration involves the use of notes that are part of the underlying diatonic scale. These notes are typically used to add melodic interest and to highlight the structure of the piece. Common diatonic decorations include trills, turns, and mordents. For instance, if the underlying diatonic scale is C major, a C major scale note can be embellished with a trill (C-D♭-C), a turn (C-D-C-B), or a mordent (C-B-C).
Chromatic decoration, on the other hand, involves the use of notes that are not part of the underlying diatonic scale. These notes can add a sense of tension or dissonance, and can be used to create a more complex and expressive melodic line. Common chromatic decorations include acciaccaturas, appoggiaturas, and grace notes. For example, given a C major scale, an appoggiatura can be added to C (A-C) or B-C (which resolves to C), an acciaccatura can be added to C (A-C), and a grace note can precede C (D♭-C).
In summary, diatonic decoration uses notes from the underlying scale, while chromatic decorations use notes that are not part of the underlying scale. Both diatonic and chromatic decorations can be used to add melodic interest and expression to a piece of music.