This paper sets out to study the letters of Gaston B., a French prisoner of war held in captivity in the camp of Münster (Germany) from the beginning of the First World War until its end. These letters make possible a relativisation of linguistic macrohistory through microhistory, by focussing on the grassroots level and by using as sources the traces of people with no significant name or identity. They shed important light on how a member of a lower class acquired the prescriptive linguistic norm through his schooling at the end of the nineteenth century and how this affected his subsequent linguistic behaviour. An individual is exposed to the political and social dimension of language planning, and his language reflects its level of success, but also reveals what grammatical tools and rules have been focused on during his schooling.