  The momentum spectrum of a periodic network (quantum graph) has a band-gap
structure. We investigate the relative density of the bands or, equivalently,
the probability that a randomly chosen momentum belongs to the spectrum of the
periodic network. We show that this probability exhibits universal properties.
More precisely, the probability to be in the spectrum does not depend on the
edge lengths (as long as they are generic) and is also invariant within some
classes of graph topologies.
