The three principal sources for Froberger's music are the following manuscripts: Libro Secundo (1649) and Libro Quarto (1656), two richly decorated volumes dedicated to Ferdinand III. Both were found in Vienna; the decorations and calligraphy are by Johann Friedrich Sautter, Froberger's friend from his Stuttgart years. Each book has four chapters and contains 24 pieces. Both include six toccatas and six suites; Libro Secundo adds 6 fantasias and 6 canzonas, whereas Libro Quarto instead has 6 ricercars and 6 capriccios. Libro di capricci e ricercate (c. 1658). 6 capriccios and 6 ricercars. Also, in 2006 an autograph manuscript was discovered (and subsequently sold at Sotheby's to an anonymus bidder), reportedly containing 35 pieces of music, 18 of which were previously unknown and remain unedited. The manuscript dates from Froberger's final years and may contain his last compositions. Three toccatas in Ms. Chigi Q.IV.25 very likely are early Froberger compositions while he studied with Frescobaldi, as Bob van Asperen has argued in 2009. Other than these, numerous manuscripts of various origin contain Froberger's music. These include the well-known Bauyn manuscript, and a very large number of less known sources, some reliable (such as the only unbowdlerized text for Méditation sur ma mort future, presumably in Weckmann's hand, or the Strasbourg manuscript of some couple of dozen of suites, possibly compiled by Michael Bulyowsky) and some not very much so. Problems arise with many of the newly discovered copies: either Froberger was constantly reworking his compositions, or the scribes were not attentive enough, but many works exist in several variants, some of which even have whole movements changed. Two standard numbering systems are used to identify Froberger's works. These are: the numbers used in the early 20th century Denkmäler der Tonkunst in Österreich series and the Guido Adler edition; commonly referred to as the DTÖ numbers or the Adler numbers. This catalogue has separate numbering for different genres, with pieces identified as Toccata No. 4, Ricercare No. 2, Suite No. 20, etc. The DTÖ contains a few compositions falsely attributed to Froberger, and some identical ones. FbWV numbers from the Siegbert Rampe catalogue compiled in the early 1990s. Rampe's catalogue is more complete and includes newly discovered pieces as well as pieces whose authorship is questioned. The Adler numbers are incorporated, for example all Toccatas are numbered 1xx, hence Adler Toccata No. 1 has the Rampe number FbWV 101. For more information,