The twenty- five surviving signed clavichords by the Hamburg makers Hieronymus and Johann Hass, covering a period of approximately forty years from 1728 to 1767, represent one of the pinnacles of musical instrument manufacture. From a study of twenty-two of these instruments a number of important clavichord design principles and constructional techniques has been determined. For instance it was found that the construction of the case joints, rack and 8ft bridge were undertaken with the aid of a template, whilst the bridge pins, tuning pins and hitchpins were positioned using a calibrated stick held perpendicular to the spine. By plotting the pin positions it has been possible to determine the method used by Hass (father and son) to space the pins in various multiples of the Hamburg Zoll or inch. Thanks to the family's habit of dating their instruments the evolution of their working methods can be discerned; in general the dimensions of the later instruments of both makers are greater than those of the earlier instruments, but remain in the same proportion. Bridge shape, rack-slot spacing and pin positioning also vary from Instrument to instrument, not because of any haphazard approach to construction but rather as the result of an intentional modification and evolution of clavichord design during the making of a large number of instruments. This research has enabled a positive identification of a clavichord in Koping, Sweden, (where the signature and date are no longer legible) as being the work of Hieronymus Hass. The techniques developed for this study can be used in the investigation of other clavichord builders and their work.