Testamentary and contractual charters of grand and feudal princes of the 15th century reflect the phonetic features of the dialects of North-Eastern Russia and some adjacent areas, including nontrivial ones – pronunciation of the preposition "k" as [x] before a fricative consonant (x svojemu), regressive assimilation of vowels (su uděla). But the most significant thing is that the charters provide new material regarding the distribution of some well-known dialectal phenomena and clarify the chronology of their occurrence or time limits of their existence. For example, it turned out that in the Tver dialect, unlike
other dialects of the center, in the 15th century soft sibilants [š’] and [ž’] were still preserved. At the same time, during this period, hard long (or complex) voiceless and sonorous sibilants have already spread in the dialects of North-Eastern Russia. Until now, the earliest of the found examples of devocalization of /v/ to [f] was considered fprok from the charter of 1501, but now the following spellings have been discovered: Vitoft in the charter dated 1402, but rewritten later in the 15th century, and hypercorrect devterem (defter’ – khan tax charter) in texts dated 1434 and 1436. Earlier it was believed that the
hardening of the affricate /c/ refers to the end of the 15th century, but now the study of charters has shown that there are earlier examples of the reflection of the hard [c], the oldest of which dates back to 1428. It follows from this that the phonetic transition of [e] to [o] after a soft consonant before a hard one, which ended before the hardening of /c/, should have been over no later than at the beginning of the 15th century. In the dialects of North-Eastern Russia, the ancient participial forms of the vynemshi type were preserved, but now they are no longer present in the dialects of this territory. Innovative constricted
forms of present, like skazyvash, developed in the dialects of the Great Russian center no later than the 15th century.