Note from Grove:

"Beethoven appears to have begun arranging folksongs for the Scottish publisher George Thomson in late 1809 or early 1810. He continued to do so at intervals until at least 1816. It was his own idea to extend the scope of the project to include songs not of British origin; most of these were never published by Thomson. In the lists below, the songs are grouped as they appear in the edition of Beethoven's complete works xxiv and have many later publications." (Grove, 409)