The paper addresses the written tradition of Bolognese from the 13th to the 19th century with a particular focus on stressed vowels. The analysis of the chosen texts has revealed noticeable divergences between graphemes and corresponding phonemes whose evolution can be traced using internal and comparative perspective. This can be explained with reference to the peculiar situation of Bologna, which has had a presence of foreign students and professors since 1088, as well as the early influence of Tuscan and Venetian models. For this reason, some dialectal features like the diphthongisation of middle-high long vowels and the palatalisation of long a remained censored in written language until the 18th-19th centuries in spite of their presence in the spoken language. Thus, the history of Bolognese should be considered a perfect example of the necessity of taking into consideration both internal and external factors in linguistic reconstruction based on written sources.