 If one is only interested in gross changes in the yield curve at a particular maturity, then one can read yields off the various datasets, using interpolation where necessary, and there is no need to model any part of the curve.If, on the other hand, one wants to describe curve movements in terms used by traders (or to extrapolate), then some form of parameterization is required.The most widely used nomenclature for describing yield curve changes uses the terms "shift", "twist" and "butterfly".