  We propose a complex network approach to the harmonic structure underpinning
western tonal music. From a database of Beethoven's string quartets, we
construct a directed network whose nodes are musical chords and edges connect
chords following each other. We show that the network is scale-free and has
specific properties when ranking algorithms are applied. We explore its
community structure and its musical interpretation, and propose statistical
measures stemming from network theory allowing to distinguish stylistically
between periods of composition. Our work opens the way to a network approach of
structural properties of tonal harmony.
