The learned men of the early Modern period were quite keenly aware that their correspondence was of interest far beyond its short-term purposes. Letters were collected, sorted and kept, in order to store the information they contained for future use, but often also with a view toward eventual publication. Between the 16th and 18th centuries, numerous collections of erudite letters appeared in print; the practice was so widespread that many letters were written with publication in mind.
The brothers Pez kept considerable sections of their passive correspondence. Of those letters which concerned their learned research and literary projects, most are extant, though not all. Other types of missives, such as the correspondence carried on by the brothers as librarians with their booksellers or the notes exchanged between Melk Abbey and the brothers at its Vienna townhouse, have survived only in minimal numbers; most of these were probably never saved for the long term.
The 18th and 19th centuries saw repeated attempts to sort and catalogue the papers left by the brothers. Near the end of the 19th century, F. Eduard Katschthaler, then archivist at Melk Abbey, in the course of preparing his fundamental article on Bernhard Pez caused the greatest part of the extant letters to be bound in three heavy folio volumes, currently kept in the Abbey Archives. Comprising some 900 letters, they make up the lion's share of the preserved correspondence. More than 100 additional letters are to be found both scattered in the archival Pez papers and bound in other manuscripts in the abbey's library.
Of the brothers' active correspondence, comparatively little remains. However, the letters of Bernhard Pez to four of his most important correspondents are preserved in significant numbers: those to the Maurist René Massuet (now in the Bibliothèque nationale, Paris), to Johann Georg Eckhart (now in Hanover), to Moritz Müller (in Einsiedeln and St. Gall) and to Johann Friedrich Schannat (in Prague). Scattered individual letters from Pez are found in Munich, Copenhagen, Metz, Rovereto or Modena, among other locations. At least a few more are likely to turn up in the course of further investigation.
The letters were often accompanied by enclosed material such as handwritten historical treatises or catalogues of manuscripts, entire printed books, drawings or engravings; letters from others were also frequently passed on. The Pez papers in Melk, making up some 15 cartons and 50 manuscripts overall, consist mostly of such enclosures and of the brothers' own notes. This material contains a great deal of valuable information, notably on the manuscript collections of numerous monastic libraries, many of which are now dispersed. While it would exceed the capacity of the project to publish all the papers in printed form, they will be digitalised and loosely catalogued over the coming years in order to make them accessible as an online image database.