Neutrinos are produced as flavor eigenstates (νₑ, νμ, ντ) but propagate as mass eigenstates. The PMNS mixing matrix rotates between bases. The two-flavor survival probability is P(νₑ→νₑ) = 1 − sin²(2θ)·sin²(1.27·Δm²·L/E), where L is in km, E in GeV, Δm² in eV².