Unlike his later works, this work is tonal, bearing the key of D minor, though it stretches this to its limit with the thoroughly extended tonality of late Romantic music, such as the quartal harmony pictured at right . It also carries a small collection of themes which appear again and again in many different guises. Besides his extension of tonality and tight motivic structure, Schoenberg makes use of another innovation, which he called "musical prose." Instead of balanced phrase structures typical of string quartet writing up to that period, he favored asymmetrical phrases that build themselves into larger cohesive groups.