In the light of the observations in the last chapter it can be seen that there is a need for certain theoretical refinements in the formal definitions of Part II. We have postponed this until we are in a position to use the tentative account of the A-deep structure of English to illustrate the need for our more complicated account. The essence of the refinement is to provide a method for getting from the shallow structure to the surface structure. It will also provide us with a definition of 'surface structure'.