Topological Crystalline Insulators (TCI) are protected by crystal symmetry, not time-reversal. In SnTe, the (001) surface hosts 4 Dirac cones (C_M = ±2) near the X̄ points, connected by the mirror line k_y=0. The model Hamiltonian H(k) = ε(k) + m(k)σ_z + λ(k_x σ_x + k_y σ_y) with m(k) = m − B(kx²+ky²). Topological phase: m < 0 (inverted). Surface states cross at k∥ on the mirror-invariant line. Breaking mirror symmetry (pressure, impurities) gaps the surface Dirac points.