structural hierarchy

A material's structure is like a set of Russian nesting dolls, or a story told at several zoom levels. Zoom all the way in and you see electrons and bonds; zoom out a little and you see atoms packed in a pattern; out more and you see grains and phases under a microscope; zoom out fully and you see the object itself. Structural hierarchy is this idea that structure exists at many nested levels at once, each level built from the one below it.

The usual ladder runs: electronic (bonding) structure, then atomic or crystal structure (how atoms pack, the unit cell), then nanostructure (features of a few to a hundred nanometers), then microstructure (grains and phases seen in a microscope, roughly micrometers), then macrostructure (features visible to the eye). A crucial fact: a property can be set at ANY of these levels. Electrical conductivity is largely electronic; stiffness is largely atomic bonding; strength is often controlled by microstructure (grain size, dislocations).

Thinking in a hierarchy is what lets an engineer target the right level. To make a metal stronger you rarely change its atoms, you refine its grains or add obstacles at the micro or nano scale. Bone and wood are famous natural examples: they are strong and tough because they are cleverly organized at every level from molecules to visible fibers, a strategy called hierarchical structure. Caveat: the levels are a useful map, not sharp walls, they blend into each other, and the boundaries (for instance where nano ends and micro begins) are conventions, not laws of nature.