 For example,
(product of a circle and the x-axis) yields smooth approximations of one half of a circle (see picture), and 
(product of two circles) yields smooth approximations of the intersection of two circles (see diagram).In CAD one uses implicit curves for the generation of blending curves, which are special curves establishing a smooth transition between two given curves.For example,
generates blending curves between the two circles 
The method guarantees the continuity of the tangents and curvatures at the points of contact (see diagram).