In contrast to systolic murmurs, diastolic murmurs are seldom innocent. The most common diastolic murmurs are due to very mild degrees of regurgitation across the aortic or pulmonic valves, when they are high-pitched and decrescendo in shape. Mitral stenosis, heard precisely over the cardiac apex, has a low-frequency, rumbling characteristic. It is best heard using the stethoscope bell applied with very light pressure, with the patient reclining to the left. On rare occasions, an atrial tumor or clot will produce a noise in diastole. Extracardiac diastolic bruits, on the other hand, may represent normal events. Examples include a venous hum, almost universally present in young people, and best heard over the anterior cervical fossa with the patient sitting and taking a deep breath. A mammary souffle audible over the breast of a nursing mother may occur as a continuous murmur (systole through diastole).