The normal curve is the familiar "bell curve:," illustrated on this page. The mathematical expression for the normal curve is y = (2×pi)^−½e⁻x2/2, where pi is the ratio of the circumference of a circle to its diameter (3.14159265…), and e is the base of the natural logarithm (2.71828…). The normal curve is symmetric around the point x=0, and positive for every value of x. The area under the normal curve is unity, and the SD of the normal curve, suitably defined, is also unity. Many (but not most) histograms, converted into standard units, approximately follow the normal curve.