It's all to do with Leibniz notation of differentials, dy and dx, etc . I've never really understood what they represent and after trying to do a Vector Analysis assignment that involved torsion of a curve c ( t ), defined only in our notes by { \ frac { d \ mathbf { B } } { ds } } = \ tau \ mathbf { N }, where B is the binormal vector, s is some sort of poorly explained arclength thing, and N is the normal vector.