Modulation refers to changing the key, moving the tonal center in a musical piece. The two primary methods of modulation are either between different tonal centers or between different tonal types.
Franz Peter Schubert (1797 - 1828) was an Austrian music composer who combined Classical and Romantic music. He would break the rules of modulation, as he preferred to suddenly change into unrelated and sudden keys. Although he didn´t use the same name specifically, he did use a form similar to the barcarolle in some of his work.
A countermelody is a piece of subordinate music that accompanies the principal one.
