In historical sociolinguistics, it is often assumed that ego-documents such as private letters represent the spoken language of the past as closely as possible. This paper will try to determine the degree of orality of seventeenth-century Dutch private letters: the degree to which the spoken local dialect is represented in these texts, and at the same time, the extent to which scribes possibly converged towards supralocal writing systems. It studies the orthographical representation of four phonemes in a corpus of letters from the provinces of Holland and Zeeland. Clear cases of local writing practices are revealed, contributing to our knowledge of the spoken language in the past, as well as to the different ways in which it was represented in written language. However, the degree to which local features appear in the corpus is remarkably low. Only a minority of the letters contains localizable features, and if a letter contains these, it is usually only in a minority of the positions which, historically, were phonologically possible. It is concluded that, in general, scribes did not aim to write their local dialect, but employed an intended supraregional variety instead.