The antinode at an open end does not form exactly at the end — it forms slightly outside at distance $e \approx 0.6r$ (where $r$ is the pipe radius). Effective length: closed pipe $L_{\text{eff}} = L + e$; open pipe $L_{\text{eff}} = L + 2e$. In resonance tube experiments, using two consecutive resonance lengths eliminates $e$: $\lambda/2 = L_2 - L_1$ (independent of end correction). End correction: $e = (L_2 - 3L_1)/2$.