More than 3900 crystalline borates, including borate minerals and synthetic inorganic borates, in addition to a wealth of industrially-important boron-containing glasses, have been discovered and characterized. Of these compounds, 99.9 % contain only the traditional triangular BO3 and tetrahedral BO4 units, which polymerize into superstructural motifs. Herein, a mixed metal K5Ba2(B10O17)(2)(BO2) with linear BO2 structural units was obtained, pushing the boundaries of structural diversity and providing a direct strategy toward the maximum thresholds of birefringence for optical materials design. B-11 solid-state nuclear magnetic resonance (NMR) is a ubiquitous tool in the study of glasses and optical materials; here, density functional theory-based NMR crystallography guided the direct characterization of BO2 structural units. The full anisotropic shift and quadrupolar tensors of linear BO2 were extracted from K5Ba2(B10O17)(2)(BO2) containing BO2, BO3, and BO4 and serve as guides to the identification of this powerful moiety in future and, potentially, previously-characterized borate minerals, ceramics, and glasses. Borates offer extended structural diversity and promise in diverse applications. Here the authors report a borate with linear BO2 units as well as NMR spectroscopy characterization that provides a quantitative basis for identification of BO2 units in polycrystalline and non-crystalline samples.