  Elucidating the atomic structure of liquid and glass is one of the important
open questions in condensed matter physics. In the conventional bottom-up
approach one starts with focusing on an atom and the short-range order of its
neighboring atoms, and the global structure is described in terms of
overlapping local clusters of atoms as building units. However, this local
approach fails to explain the strong drive to form the medium-range order which
is distinct in nature from the short-range order. We propose an even-handed
scheme with an additional top-down approach. In the top-down approach one
starts with a high-density gas state and seeks to minimize the global potential
energy through density waves. The local bottom-up and global top-down driving
forces are not compatible, and the competition and compromise between them
result in a final structure with the medium-range order. The density waves are
pinned to atoms through the phase factors and amplitudes which reflect atomic
dynamics. This even-handed approach provides a more intuitive explanation of
the structure of simple liquid and glass and its relation to properties of
liquid, such as viscosity and fragility.
