This chapter deals with a number of important considerations in design, many of a rather detailed nature. It looks at the way parts are fitted and joined together, derives some principles about the design of mating surfaces or abutments, clarifies the function of screw threads and broaches the important subjects of register and sealing. Some of the influences which bear on the designer in these areas relate to the structural aspects considered in the previous chapter, but others derive from manufacture and assembly, or from functional requirements other than sustaining load.