The research aims to identify the specific characteristics of the implementation of punctuation trends in natural written language as exemplified by the use of separating punctuation marks in a corpus of Russian epistolary texts written by semiliterate authors. The research is novel in that it is the first one to compare punctuation trends found in different genres of natural written language. Moreover, for the first time, the functions of separating punctuation marks in the middle and end of sentences in letters written by semiliterate authors were considered in relation to the main punctuation trends of natural written language. The research findings showed that most of the main punctuation trends of natural written language (economy, interchangeability, and universalization of punctuation marks, actualization of communicatively significant parts of the sentence (parcellation), individual free combination of marks) are realized when using separating punctuation marks in the middle of a sentence (comma, semicolon, colon, dash) and at the end of a sentence (period, exclamation mark) in letters written by semiliterate people; at the same time, the realization of natural written language trends does not depend on the productivity of the marks in the corpus. The most productive and active in realizing the communicative trends of natural written language are basic marks, a period and a comma, however, natural written language trends are also realized through other, less productive separating punctuation marks. As a result of the research, it was also found that the tendency toward unification of the functions of marks, characteristic of natural written language, is not realized in the corpus through separating punctuation marks due to their multifunctionality.