A sound change in a medieval language is usually dated from the first manuscripts that spell formerly distinct vowels interchangeably. In one region, official charters maintain the distinction until 1320. Private letters from the 1290s, however, repeatedly substitute the two vowel symbols in unstressed words but preserve them in carefully copied names. Poems composed locally before 1300 rhyme words from the two historical vowel classes, while poems imported from another region do not. Scribes of the charters were trained from conservative formularies that continued to prescribe the older spelling after it had disappeared from some spoken dialects.