the length scales of structure
'Structure' in materials science means how the pieces are arranged - but pieces at what size? A material has structure at many zoom levels at once, like a map you can view from orbit all the way down to a single street. Each level of magnification reveals different features, and each level affects the material's behavior.
The ladder of scales, from smallest up. Subatomic: electrons and the nucleus inside a single atom (this sets bonding). Atomic: how atoms are arranged relative to their neighbors - the crystal structure, or lack of one, at roughly 0.1 to 1 nanometer. Nanostructure: features from about 1 to 100 nanometers (fine particles, thin layers). Microstructure: what you see under a microscope, roughly 0.1 to 1000 micrometers - grains, phases, boundaries. Macrostructure: what you see with the naked eye, millimeters and up - bulk shape, large defects, weld seams. For scale, a human hair is about 70 micrometers across, and an atom is roughly a million times smaller than that.
Properties are decided at whichever scale controls the behavior - and often it is the small ones. Strength is controlled by atomic-scale dislocations and micrometer-scale grains; conductivity by the subatomic electron arrangement. The central skill of materials science is knowing which length scale to look at for the property you care about, and processing the material to get the right structure there.
Zoom into a steel paperclip: to the naked eye, a smooth gray wire (macro); under a microscope, a patchwork of crystal grains about 10 micrometers across (micro); deeper, atoms stacked in a repeating cubic lattice (atomic); deeper still, the electron sea that makes it a metal (subatomic). One object, four structures.
The same steel, seen at four levels of zoom.
There is no single 'the structure' of a material - always ask 'at what length scale?' A metal can look uniform to the eye yet be a busy patchwork of grains and phases under a microscope, and that hidden microstructure usually matters most.