The "fluoride route" of zeolite synthesis was pioneered by Flanigen and Patton of Union Carbide, who, in 1976, filed a patent describing the hydrothermal synthesis of sizeable single crystals of MFI-type Silicalite-1 in the presence of ammonium fluoride. Subsequent research by a number of groups showed that the addition of fluoride does not only aid the preparation of large and defectfree crystals, but that it may also lead to the formation of new framework types or distinct framework compositions that are not synthetically accessible in the absence of fluoride. The role of fluoride is particularly important in the synthesis of pure-SiO2 (all-silica) zeolites: In these neutral-framework materials, the fluoride anions balance the charge of the cationic organic structure-directing agents (OSDAs) that are added to promote the formation of a particular structure type (although some all-silica zeolites can be synthesized in the absence of fluoride, the crystals tend to have a rather high concentration of charge-balancing defects ). Like the OSDAs, which are encapsulated in larger cages or channels, the fluoride anions are incorporated in the crystal structures of the as-synthesized zeolites, and they can be localized with diffraction methods. If double four-ring (d4r) units are present in the structure, the fluoride anions are located at or near the centre of these cube-like cages. In structures without d4r cages, the fluoride anions reside in other small cages, where they are bonded to a silicon atom at one of the corners of the cage, leading to the formation of trigonal-bipyramidal [SiO4F] -units. The first observation of such a building unit in the crystal structure of an as-synthesized all-silica zeolite was reported in 1995 for nonasil (NON framework type code 10 ), followed by analogous findings for various other zeolites. The body of crystal structure data shows a preference of fluoride to bond to Si atoms that are part of small rings, most typically four-membered rings (4MRs) or, if these are not present in the structure, 5MRs.