Can you read the NTE mechanism off the crystal structure? Structural fingerprints (occupancy-weighted Maxwell floppiness f=(3n−m)/3n with ASE covalent radii +0.40 Å cutoff, mean coordination, 2-coordinate linker fraction, packing openness, density) for 9 known negative-thermal-expansion materials spanning 5 mechanisms (rigid-unit modes, transverse linker modes, ferroelectric, magnetovolume, electronic/Mott) and 3 positive-expansion controls (dense ionic, dense metal, floppy framework). Structures from COD (cod_id column) or built from prototypes ('built'); all CIFs passed the structure sanity card v4.4.