Antiferromagnetic Ising spins on a triangular lattice cannot all satisfy their interactions simultaneously — geometric frustration. The ground state is massively degenerate with residual entropy S₀ ≈ 0.3231 Nk_B (Wannier 1950), and no ordered phase exists at T > 0.
The triangular lattice antiferromagnet was solved exactly by Wannier (1950): no phase transition occurs for T > 0, and the residual entropy per spin is (3/2)ln(4/3) ≈ 0.431 — or equivalently, for the triangular lattice S₀/Nk = 0.3231. This "spin liquid" behavior is the classical precursor to quantum spin liquids. Each triangle has exactly one unsatisfied bond (colored orange), and there is no way to eliminate all frustration simultaneously.