Vortex lattice melting in untwinned and twinned single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

The melting transition in twinned and untwinned single crystals is measured resistively in fields up to 8 T as a function of the angle between the c axis and the a-b plane. The angular dependence follows the Lindemann criterion with ${\mathit{c}}_{\mathit{L}}$=0.15. The suppression of melting by strong pinning by twin boundaries is demonstrated.