  Graphene is a gapless semiconductor in which conduction and valence band
wavefunctions differ only in the phase difference between their projections
onto the two sublattices of the material's two-dimensional honeycomb crystal
structure. We explain why this circumstance creates openings for broken
symmetry states, including antiferromagnetic states in monolayer and bilayer
graphene and exciton condensates in double-layer graphene, that are momentum
space analogs of the real-space order common in systems with strong local
interactions. We discuss some similarities among, and some differences between,
these three broken symmetry states.
