In a 1789 treatise, the Darmstadt musician J.G. Portmann presents what amounts to a multi-level harmonic analysis of Mozart’s Sonata for Piano in D, K. 284, I. Portmann’s interpretation of the movement's exposition is in line with concepts expressed by other eighteenth-century theorists, but suggestively differs from standard modern conceptions of the form, especially in its understanding of what nowadays is labeled as the secondary theme.