Thermal expansion. The vast majority of dense inorganic materials feature positive thermal expansion (PTE), fundamentally due to the asymmetry of 2-body interactions around their minimum: as temperature increases, the bonds tend to elongate rather than contract. While some dense materials display negative thermal expansion (NTE), their number is limitedthey are the exception, rather than the norm. The microscopic mechanisms behind their anomalous thermal behavior have been explained in various ways, but usually involve the concerted motions of the constituents of their molecular structure, or the activation of low-frequency vibration modes. Metal-organic frameworks, covalent organic frameworks and other supramolecular assemblies, on the other hand, tend to display NTE more frequently. Many supramolecular frameworks are built from M-ligand-M linear linkages, where the linker is aligned with the M• • • M axis. Thermal motions then enable the out-of-axis low-energy vibration modes of the linkers, which on average tends to shorten the M• • • M distance, and therefore increase the framework density. One can wonder whether NTE is not, indeed, the norm in the world of supramolecular frameworks -although we are not aware of any systematic study so far.