在三次样条中，要寻找三次多项式，以逼近每对数据点间的曲线。在样条术语中，这些数据点称之为断点。因为，两点只能决定一条直线，而在两点间的曲线可用无限多的三次多项式近似。因此，为使结果具有唯一性。在三次样条中，增加了三次多项式的约束条件。通过限定每个三次多项式的一阶和二阶导数，使其在断点处相等，就可以较好地确定所有内部三次多项式。此外，近似多项式通过这些断点的斜率和曲率是连续的。然而，第一个和最后一个三次多项式在第一个和最后一个断点以外，没有伴随多项式。因此必须通过其它方法确定其余的约束。最常用的方法，也是函数spline所采用的方法，就是采用非扭结(not-a-knot)条件。这个条件强迫第一个和第二个三次多项式的三阶导数相等。对最后一个和倒数第二个三次多项式也做同样地处理。

-the Cubic Spline, to find three polynomial, approaching every right to point the data curve. The kind of terms, these data points called breakpoints. Because only two points a straight decisions, and in between the two curves can be used an infinite number of cubic polynomial approximation. Therefore, in order for the outcome of uniqueness. The Cubic Spline, add the cubic polynomial bound by the conditions. By limiting each cubic polynomial of the first order and second-order derivative, it breakpoint in the same Department, we can better identify all internal cubic polynomial. In addition, the polynomial approximation of these breakpoints slope and curvature is continuous. However, the last one and a cubic polynomial in a breakpoint and the final one, not accompanied by polynomials. There