The National Edition was based on sources originated from the composer, mainly autographs, copies of autographs and first editions with the composer's corrections, and pupils' copies with Chopin's annotations. In cases when original sources were lacking, the closest possible materials were used. Collecting the source materials was a laborious task which took years of effort. The characteristics of sources, the links and discrepancies between them as well as the reasons for particular editorial decisions are discussed in the Source Commentary in each volume. The Performance Commentary appended to each volume includes: the realization of ornaments, comments on pedal markings (the original markings sometimes are inadequate, due to the difference in sound between pianos used in Chopin`s times and modern pianos),