  The relative orientation (twist) of successive layers of stacked
two-dimensional (2D) materials creates variations in the interlayer atomic
registry. The variations often form a super lattice, called a moir\'e pattern,
which can alter electronic properties. In this work we introduce a
classification of the single-particle electronic structures that can occur in
twisted stacks of 2D layers by characterizing them as "moir\'e molecules" or
"moir\'e crystals". The molecules generate localized electronic states and
moir\'e flat bands, while the crystals are sometimes unconventional and produce
electronic banding in the configuration basis. The underpinning of this
classification is the duality between interlayer configuration and monolayer
Bloch momentum in moir\'e Hamiltonians. We apply this understanding to diagrams
of local electron density in untwisted geometries to produce intuitive and
quantitative predictions of twistronic properties. We provide a conceptual
introduction to this framework through a one-dimensional model, and then apply
it to 2D twisted bilayers of the semi-metal graphene, and of MoS$_2$, a
representative material of the transition metal dichalcogenide (TMDC) family of
semiconductors. This level of thorough understanding of twistronic phenomena is
vital in the search for new material platforms for localized moir\'e electrons.
