In the next three chapters we shall sketch the development of our Old Northwest Chinese (ONWC) sound system, first to the Sui-Tang Chang’an (STCA) stage, thence through mid-Tang (MTCA) to the approximately concurrent Shazhou (SZ) and Late Tang Chang’an (LTCA) periods, and finally to the modern dialects. Our reconstructions for the pre-modern stages have been presented in a number of articles and a monograph, all cited in the bibliography. In several cases, to be noted as they arise, these earlier reconstructions will be revised and amplified in various ways here. Our approach will be to take our reconstructed forms as given starting points for the historical discussion. This is done for convenience of presentation and should not imply that the reconstructions in question are generally accepted or uncontroversial. Direct reference will be made to the data in Part III below by giving the appropriate entry numbers. Where necessary for clarity these numbers are enclosed in square brackets.