Prandtl's mixing-length theory predicts that near a wall, turbulent mean velocity follows a universal logarithmic profile: U⁺ = (1/κ)ln(y⁺) + B. The viscous sublayer (U⁺ = y⁺) transitions to the log-law at y⁺ ≈ 30. This "law of the wall" holds across Reynolds numbers from 10³ to 10⁸.