Boron's electron deficiency favors multicenter bonding and enables planar or quasiplanar boron nanostructures with unusually stable pristine edges and corners. Yet a compact bonding picture that explains these stability trends remains elusive. Here we introduce a bonding model for triangular-lattice-based planar boron clusters, defined by four bonding rules that use only 3c-2e and 2c-2e bonding and σ bond resonance. The model allows intuitive construction of σ bonding configurations without extensive quantum calculations, revealing the origin of the high stability of planar boron clusters and rationalizing their effective self-termination at pristine edges and specific corners. From this model we extract three stability criteria: energetically favored clusters tend to have (i) obtuse-angle convex and/or concave corners, (ii) flat edges with an even number of atoms, and (iii) an increasing number of hexagonal holes as the cluster grows. Applying these criteria, we predict ${\mathrm{B}}_{49}, {\mathrm{B}}_{70}$, and ${\mathrm{B}}_{84}$ as particularly stable planar clusters beyond the well-known ${\mathrm{B}}_{36}$, reproducing density-functional theory trends in formation energies. Independent validation using a machine learning force field–accelerated global structure search across both planar and three-dimensional manifolds confirms these predictions, while energy gaps, in this size range, remain small enough that planar forms stay competitive and are further stabilized on substrates. This resulting framework provides fundamental insights into boron chemistry and a practical route to rational design and scalable prediction of boron-based nanostructures.

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• Chemical bonding
• Nanoclusters
• Triangular lattice
• Density functional theory
• Machine learning