The scaling of ranks of organ flue pipes to produce a coherent, characteristic, and tonally balanced ensemble is a problem which has confronted organ builders for many centuries, but the temperament used to tune these pipes has likewise been of concern. With an analysis of the Fantasia Chromatica by Jan Pieterszoon Sweelinck, and its contrapuntal fabric, the proposition by Simon Stevin of “finding 11 mean proportional parts between 2 and 1, learned through the 45th proposition of my French arithmetic” serves as a solution for a modification in temperament of the organ pipes at the Oude Kerk in Amsterdam where it was performed by Sweelinck.