Voice leading is how individual melodic lines (voices) combine and move as they pass from chord to chord. Perfect voice leading makes each voice singing and natural, and their combination transparent and euphonious, even when the listener is unaware of this “underwater” work.
Classical theory developed strict rules: avoid parallel fifths and octaves, move voices smoothly (ideally by the smallest distance), and resolve dissonances properly. It is precisely flawless voice leading that is the secret of the “magic” of J.S. Bach's chorales, where four voices flow like four independent melodies and at the same time create a perfect harmony. It is one of the cornerstone skills of a composer and arranger of any style.
