The paper defines the elementary principles for creating an electronic corpus of Serbian medieval charters and letters. The commitment to the principle of maximum representativeness of the corpus of medieval charters and letters, determined entirely by the preserved written legacy (based on manuscripts, microfilms or photographs), excludes the indispensability of applying the principle of balance, while simultaneously satisfying the principle of reliability, since charters and letters known solely by the edition are not included in the corpus. The selection of texts is done according to the diplomatic criterion by excluding the transcripts and copies of documents already available in the original, as well as later transcripts, chronologically and linguistically distant from the assumed original. This approach to the selection of texts is justified by the size of the corpus, as well as by the exceptional cultural and historical significance of medieval charters and letters. The definition of the metadata about corpus texts is determined by their general diplomatic properties, as well as the corpus search needs for diatopic, diachronic and genre variations. Conversion of texts into electronic form strives for fidelity to the original, encompassing the preservation of abbreviations, superscript letters and original punctuation, as well as the absence of accent marks and contemporary rules of capitalization.