[Help-gsl] what does xstride,ystride mean and how to set them?

[Richard]

 Hi,
I want to use  gsl_fit_linear function. But I am puzzled withe the 
parameters xstride and ystride.
Could anybody like to give some explanations ?
Thanks in advance.

Richard
PS. function signature :
 int *gsl_fit_linear* (const double * x, const size_t xstride, const 
double * y, const size_t ystride, size_t n, double * c0, double * c1, 
double * cov00, double * cov01, double * cov11, double * sumsq)