While tone clusters are conventionally associated with the piano, and the solo piano repertoire in particular, they have also assumed important roles in compositions for chamber groups and larger ensembles. Robert Reigle identifies Croatian composer Josip Slavenski's organ-and-violin "Sonata Religiosa" (1925), with its sustained chromatic clusters, as "a missing link between Ives and ." Bartók employs both diatonic and chromatic clusters in his Fourth String Quartet (1928). The sound mass technique in such works as Ruth Crawford Seeger's String Quartet (1931) and Iannis Xenakis's "Metastaseis" (1955) is an elaboration of the tone cluster. "Unlike most tonal and non-tonal linear dissonances, tone clusters are essentially static. The individual pitches are of secondary importance; it is the sound mass that is foremost." In one of the most famous pieces associated with the sound mass aesthetic, containing, "one of the largest clustering of individual pitches that has been written", Krzysztof Penderecki's "Threnody to the Victims of Hiroshima" (1959), for fifty-two string instruments, the quarter-tone clusters "see to have abstracted and intensified the features that define shrieks of terror and keening cries of sorrow." Clusters appear in two sections of the electronic music of Stockhausen's "Kontakte" (1958–60)—first as "hammering points...very difficult to synthesize", according to Robin Maconie, then as glissandi. In 1961, Ligeti wrote perhaps the largest cluster chord ever—in the orchestral "Atmosphères", every note in the chromatic scale over a range of five octaves is played at once (quietly). Ligeti's organ works make extensive use of clusters. "Volumina" (1961–62), graphically notated, consists of static and mobile cluster masses, and calls on many advanced cluster-playing techniques.