The sign change method is used to find a starting value for an iteration before performing the iteration . If f(x) = 0 is the equation being solved, two values of x are found such that when they are substituted into f(x) one of them gives f(x) < 0, the other gives f(x) > 0. Thus it is known that the solution lies between the two values of x used (providing their is continuity of the function).