  We introduce a minimum tight-binding model with only three parameters
extracted from graphene and untwisted bilayer graphene. This model reproduces
quantitatively the electronic structure of not only these two systems and bulk
graphite near the Fermi level, but also that of twisted bilayer graphene
including the value of the magic angle, at which bands at $E_F$ flatten without
overlap and two gaps open, one above and one below $E_F$. The Hamiltonian is
sufficiently transparent and flexible to be adopted to other twisted layered
systems.
