The territorial factor in language diachrony studies is gaining more and more importance. Having appeared at a certain time in the language space, writing monuments retain their ability to be localized in it, therefore they not only become sources for studying the language at a certain synchronous level, but also create conditions for the study of language evolution. This is especially evident at the level of the language dictionary. By involving the lexical material of written monuments from a specific language (dialect) area in the historical dictionary, the lexicographer gets the opportunity to use the potential of the geographical projection of the word to determine or clarify the semantics, interpret changes in it, relying on the dictionaries of modern dialects and living speech. Genre- and structurally diverse texts from Polissia are a representative part of the source base of the «Dictionary of the Ukrainian Language of the 16th – first half of the 17th Centuries». Distinguished by relative democracy in the language expression, based mostly on the living lexicon of spoken language, these texts supplemented the register of the dictionary with uniquely fixed lexical units that have broad and understandable contexts. These are the commonly used vocabulary of the most diverse thematic groups (the names of realities from everyday life of the time are especially valuable for the dictionary), and professional names, and administrative-legal, religious, philosophical, botanical, etc. terminology. The onymicon of the vocabulary of monuments from Polissia reveals a peculiar tradition of names of persons and places, sheds light on the lingual picture of the world of the Polissia people. The more texts a lexicographer works with, the more complete the linguistic representation of the historical period will be in the register and in the illustrative part of the dictionary.