Thematic-index numbers used in uniform titles for keyboard works are those given by Gustav Leonhardt in Sweelinck's Opera omnia, v. 1 (Amsterdam, 1968- ); e.g., Fantasias, keyboard instrument, L. 34a
His The organ works, p1979 label (J.P. Sweelinck) prog. notes (Yan Pīterusuzōn Suvērinku)
Grove music online, Aug. 14, 2006 (Sweelinck (Swelinck, Zwelinck, Sweeling, Sweelingh, Sweling, Swelingh), Jan Pieterszoon; b. ?May 1562, Deventer, d. Oct. 16, 1621, Amsterdam; Dutch composer, organist, and teacher)