The second heart sound is produced by the closure of the aortic and pulmonary valves and appears at the end of ventricular systole and is most clearly auscultated at the left and right margins of the sternum between the 2nd ribs. In hypertensive aortic dilatation, the second heart sound is enhanced, in aortic stenosis the second heart sound is diminished, in pulmonary valve hypertension and pulmonary artery dilatation the second heart sound is hyperactive, and in pulmonary stenosis the second heart sound is diminished. Under normal circumstances the second heart sound appears to split during inspiration and overlap during expiration. Second heart sound splitting can be physiological or pathological. Pathological is most commonly seen in atrial septal defect, pulmonary stenosis, and right heart failure, where its splitting is usually fixed. Irregular splitting can be seen in complete right bundle branch block, pulmonary hypertension, right heart insufficiency, pulmonary valve stenosis, pulmonary artery dilation, severe mitral regurgitation, ventricular septal defect, and left ventricular pre-excitation. There are also paradoxical splits, which are mainly seen in complete left bundle branch block, atherosclerosis, left heart insufficiency, aortic dilatation, embolism, and right ventricular preexcitation.