What do you mean "the harmony"? That is extremely unspecific.
It's not so unspecific in fact. Do you know what's "harmony"? If you don't, let me explain: In a song, there are staffs for
melody and staffs for
harmony. Generally, the melody is the main staff of the song (but there may be more than one staff for this, specially if you combine two instruments for it), and the rest is percussion and harmony. In most piano songs, the right hand plays the melody and the left one plays the harmony, but a few songs are inverted.