Primary Sources are directly related to the subject being covered, or firsthand accounts of the subject. As such, what sources are primary and secondary are obviously different for each subject. An example of a primary source for Franz Schubert, for example, might be a collection of letters from or to Schubert, or an interview with one of Schubert's friends.

Being as close as possible to the core of the subject matter, primary sources are commonly most useful for gathering firsthand facts or information about the subject. Because they are firsthand, not derived, they are not yet analyzed from the outsider's perspective, and as such often require more in-depth study to absorb the information.

Secondary Sources are not directly related to the subject being covered--sources that are derived from that which is primary. So a commentary or research paper studying the letters of Schubert is considered a secondary source in Schubert research. The world of information is an endless pool of primary and secondary sources, wherein a book by Heinrich Schenker studying Beethoven's ornamentation would be a secondary source for research on Beethoven, but a primary source for a study of Schenker's writings.

Secondary sources are more distant from the core of the subject, and as such the original information can be distorted. However, they approach the subject from an outsider's perspective and therefore are useful for evaluation of the subject from a different point of view.