Historical sociolinguistics relies on sociolinguistic insights in the analysis of various texts from the past in order to reconstruct the way in which society induces variability in language and how this is reflected in language use. It has only recently become possible to record speech, which is why letters, being the means of communication that is the closest to speech, are the most suitable type of texts for a historical sociolinguistic analysis. This study aims to provide a historical sociolinguistic analysis of the 20th century language in Varaždin based on Oskar Szabo's private letters. It is based on theories of language and identity resulting form classical sociolinguistics research. By means of the analysis of Oskar Szabo's language and the content of his letters, I described his language use, but also the ideology influencing his attitude towards language. Finally, I described how linguistic features and the choice of language codes contribute to the construction of Szabo's identity. Using the example of the idiolect of an individual (which is, in this case, learned but not socially prominent), I also provided insight into certain aspects of social and cultural reality of Szabo's time.