  In Raman spectroscopy of graphite and graphene, the $D$ band at $\sim
1355$cm$^{-1}$ is used as the indication of the dirtiness of a sample. However,
our analysis suggests that the physics behind the $D$ band is closely related
to a very clear idea for describing a molecule, namely bonding and antibonding
orbitals in graphene. In this paper, we review our recent work on the mechanism
for activating the $D$ band at a graphene edge.
