The complete correspondence of Robert and Clara Schumann is estimated at 20,000 letters. In times before the telephone or email, the letter was the essential means of long-distance communication. Robert Schumann collected and filed the majority of letters addressed to him. These 28 stapled volumes of 5,500 letters were received by the State Library in Berlin from the estate of Clara Schumann. However, they were moved during the Second World War to Silesia and are today housed in the in the Biblioteka Jagiellonska in Krakow/Poland.
The Düsseldorf Robert Schumann Research Institute, which is involved in the Collected Works of Schumann and is supported by the Academies programme, had the entire Krakow letters, the so-called »Correspondence«, filmed; it also secured the rights to their publication, made transcripts of these 5,500 letters and entered them into databases. In addition, all the discovered letters of Schumann have been entered into a database and the handwritten index, housed in Zwickau, of the circa 2,400 letters posted by Schumann has been transcribed in its entirety. All of this groundwork, including a keyworded literature database, was made available to the Zwickau Letters Project by the Düsseldorf Schumann Research Institute. Also, the commentary on the letters' text has been and continues to be supported by the Düsseldorf Research Institute.
The Robert Schumann House in Zwickau houses Schumann's own handwritten letter indexes in which he carefully listed letters written and received by him, often with a note as to their contents. The world's largest collection of letters by Robert and Clara Schumann is also to be found in the Zwickau archive: just under 300 letters by Robert and more than 2,000 letters by Clara Schumann.
Indeed, the first Schumann biography by Wilhelm Joseph von Wasielewski in 1858 featured 70 letters by Robert Schumann from the years 1833-1854 in a separate appendix. By 1904 other important editions had appeared, some supervised by Clara Schumann herself or commissioned by her: Jugendbriefe, ed. by Clara Schumann. Leipzig 1885, 4th [edited] edition 1910; Briefe. Neue Folge, ed. by Gustav F. Jansen. Leipzig 1886, 2nd [substantially altered] edition 1904; Hermann Erler, Robert Schumanns Leben aus seinen Briefen geschildert. Berlin 1886. However, they offer only a fraction of the inventory of letters verifiable today, and the letter texts are sometimes reproduced in abridged form. Curiously, the most comprehensive edition of Schumann letters published thus far, with some 1,100 letters, is a Russian translation of the now deceased Moscow musicologist Daniel W. Shitomirski. More recent German-language editions which obey modern editing principles were dedicated to the letters contained in individual collections, such as those of the Bonn University Library (ed. by Siegfried Kross, Bonn 1978/exp. 1982) and the Bonn City Archive (ed. by Thomas Synofzik, Bonn 1993).