This article reproduces correspondence between Georg Rasch of The University of Copenhagen and Benjamin Wright of The University of Chicago in the period from January 1966 to July 1967. This correspondence reveals their struggle to operationalize a unidimensional measurement model with sufficient statistics for responses in a set of ordered categories. The article then explains the original approach taken by Rasch, Wright, and Andersen, and then how, from a different tack originating in 1961 and culminating in 1978, three distinct stages of development led to the current relatively simple and elegant form of the model. The article shows that over this period of almost two decades, the demand for sufficiency of a unidimensional parameter of the object of measurement, which enabled the separation of this parameter from the parameter of the instrument, drove the theoretical development of the model.