This paper examines two letters and two visit cards written between 1904 and 1908 by Ferdinand de Saussure to his colleague Max Niedermann. This correspondence, so far unpublished, is preserved in the Niedermann Fund of the Public and University Library (BPU) in Neuchâtel. The analysis focuses mainly on the letter written on the 18^th of July 1904, where Saussure addresses, albeit briefly, some methodological issues on the correct formulation of phonetic changes, anticipating to Niedermann a crucial aspect of his theoretical thought, i.e. the distinction between the synchronic and diachronic order.