The Witch of Agnesi (or versiera) is a curve named for Marie Agnesi who first studied it. It may be defined geometrically as follows: In a circle with centre at and diameter a (hence tangent to the x-axis at the origin), a secant through the origin is drawn, intersecting the circle in A and the line y = a in B. The intersection of lines through A parallel to the x-axis and through B parallel to the y-axis determines a point P. The Witch of Agnesi is the locus of the point P as the angle the secant makes with the x-axis assumes all values from 0° to 180°.