Block-spin renormalization (Kadanoff 1966): coarse-grain by replacing b×b blocks with single spin (majority rule). At T_c ≈ 2.269, the system looks statistically the same at all scales (scale invariance = fixed point of RG flow). Below T_c: flows to ordered fixed point. Above: flows to disordered. Effective coupling J' = J·b^(1/ν) near critical point.