A new orthorhombic polar variety of Pb-hilgardite containing Na⁺ and HOCl²⁻ ions in the cavities of the framework has been prepared by hydrothermal synthesis. A correlation between the structural features and nonlinear-optical properties of the crystals is discussed. Similar to the hexaborate group, pentaborates with different degrees of condensation of the boron—oxygen radical are characterized by close structural similarity, which is described within the _OD theory with distinguishing structural units (blocks or modules) of different dimensionality—bricks, rods, and layers. Consideration of the variants of their connection allows us to describe the diversity of the pentaborate family including hypothetical structures.