Step up the hierarchy on a real protein: the linear chain (primary), the local hydrogen-bonded helices and sheets (secondary), one folded subunit (tertiary), and the full symmetric assembly (quaternary).
Structures from the PDB:
deoxyhemoglobin 2HHB (Fermi & Perutz) and triosephosphate isomerase
1TIM (Banner et al.). Secondary structure is assigned by 3Dmol from the coordinates
and colored by type (helix magenta, sheet yellow, loop white); the tertiary view isolates the
first chain and any bound heme; the quaternary view colors each subunit separately. Viewer built
with 3Dmol.js.