In describing the form of his Fourth String Quartet, Bartók employed the terminology of the German Formenlehre with which he was acquainted thanks to the teaching of Hans Koessler. Bartók's elucidation suggests that the models for the construction of thematic units, contrasts, developments and closing segments, which were normative for the sonata cycle in the nineteenth century, continued to exercise their influence despite the distance from functional tonality. An analysis of the five movements of the Fourth String Quartet with systematic reference to Bartók's synopsis shows that a subtle dialectic between adoption and transformation was at stake. The principles of traditional form gained new strength through the reinvention of harmonic structures, melodic lines and hypermetric groups. The symmetrical–architectural framework was preserved but filled with unconventional tone progressions, irregular phraseology, and a new sense for development and reprise.