In 1893 Hugo Riemann published his Vereinfachte Harmonielehre, simplified harmony. He traced the chords of a key back to three functions: Tonika, Dominante and Subdominante, written T, D and S. Each chord is read as a form of one of the three.
Riemann’s function theory is still used to teach harmony in German-speaking countries. A sample exam in music theory, published by the University of Music and Theatre in Rostock for applicants to music teacher training, asks for voice leading such as D7 – T, and for an extended cadence with secondary functions.