One crucial change was the shift towards harmonies centering on "flatward" keys: shifts in the subdominant direction . In the Classical style, major key was far more common than minor, chromaticism being moderated through the use of "sharpward" modulation (e.g., a piece in C major modulating to G major, D major, or A major, all of which are keys with more sharps). As well, sections in the minor mode were often used for contrast. Beginning with Mozart and Clementi, there began a creeping colonization of the subdominant region (the ii or IV chord, which in the key of C major would be the keys of d minor or F major). With Schubert, subdominant modulations flourished after being introduced in contexts in which earlier composers would have confined themselves to dominant shifts (modulations to the dominant chord, e.g., in the key of C major, modulating to G major). This introduced darker colors to music, strengthened the minor mode, and made structure harder to maintain. Beethoven contributed to this by his increasing use of the fourth as a consonance, and modal ambiguity—for example, the opening of the Symphony No. 9 in D minor.