  We prescribe general rules to predict the existence of edge states and
zero-energy flat bands in graphene nanoribbons and graphene edges of arbitrary
shape. No calculations are needed. For the so-called {\it{minimal}} edges, the
projection of the edge translation vector into the zigzag direction of graphene
uniquely determines the edge bands. By adding extra nodes to minimal edges,
arbitrary modified edges can be obtained. The edge bands of modified graphene
edges can be found by applying hybridization rules of the extra atoms with the
ones belonging to the original edge. Our prescription correctly predicts the
localization and degeneracy of the zero-energy bands at one of the graphene
sublattices, confirmed by tight-binding and first-principle calculations. It
also allows us to qualitatively predict the existence of $E\ne 0$ bands
appearing in the energy gap of certain edges and nanoribbons.
