The GHP formalism notices that given a spin-frame ( o A , ι ι A ) with o A ι ι A = 1 , the complex rescaling ( o A , ι ι A ) → → ( λ λ o A , λ λ − − 1 ι ι A ) does not change normalization. The magnitude of this transformation is a boost, and the phase tells one how much to rotate. A quantity of weight ( p , q ) is one that transforms like η η → → λ λ p λ λ ¯ ¯ q η η . One then defines derivative operators which take tensors under these transformations to tensors. This simplifies many NP equations, and allows one to define scalars on 2-surfaces in a natural way.