Double four-ring cages (d4r units, face symbol 4 , t-cub tile in the nomenclature of natural tilings ) are a prototypical building unit of zeolites and zeotypes. A recent statistical analysis of the zeolite frameworks included in the IZA (International Zeolite Association) Database of Zeolite Structures showed that the t-cub tile is the second most frequent tile, occurring in 36 out of 239 zeolite frameworks. As d4r units consist of four-membered rings, the T-O-T angles (where T = tetrahedrally coordinated atom such as Si, Ge, Al, P, …) along the edges are relatively small, typically below 140 degrees. Because the equilibrium Si-O-Si angle is closer to 150 degrees, these building units are strained in all-silica zeolites. The strain can be reduced through an incorporation of heteroatoms, especially germanium, at some corners of the cage, or through encapsulation of fluoride anions inside the cages. In fact, many all-silica zeolites containing d4r units have, so far, not been obtained in the absence of fluoride. While the formation of a particular framework type is primarily governed by the organic structure-directing agents (OSDAs), which are encapsulated in larger cavities, this indicates that the fluoride anions play an important structure-directing role in the formation of d4r cages. Examples of d4r-containing all-silica zeolites include octadecasil (AST framework type in the IZA nomenclature ), ITQ-7 (ISV), ITQ-12 (ITW), ITQ-13 (ITH), ITQ-29 (LTA), HPM-1 (STW), and IM-17 (UOV). Some of these neutral-framework materials have been proposed, for example, for applications in adsorption-based gas separations (ITQ-12: propene/propane separation; ITQ-29: carbon dioxide/methane separation ) and in hydrogen-selective membranes, for the storage of mechanical energy through water intrusion/extrusion, and as low-κ dielectrics. Synthesis in the presence of fluoride is also widely used in the field of germanosilicates, where various extra-large pore zeolites containing d4r units have been reported