INTRODUCTION In cardiac resynchronization therapy, pacing the left ventricle (LV) at sites of prolonged electrical delay is associated with better outcomes. We sought to characterize the interrelationships between intrinsic, right-ventricular (RV)-paced, and LV-paced interventricular delays. METHODS AND RESULTS The following electrical timings were measured at implantation for all electrodes of the LV quadripolar leads: QLV, interventricular delay in intrinsic rhythm (RVs-LVs), in RV-paced rhythm (RVp-LVs), and in LV-paced rhythm (LVp-RVs). We included 32 patients (78% men, age 72 years, LV ejection fraction 29%, left bundle branch block 84%). QLV and RVs-LVs were correlated (R2 = .72, p