Almost all the relations discussed above make no reference to the possibility that--even in the absence of common-practice tonal conventions--works may project a focus on certain pcs, privileging them above other members of the aggregate. It is clear, however, that a good deal of 20th-century music does indeed project what is sometimes called pc centricity; the music of Stravinsky, Bartók, and Debussy comes easily to mind here. This focus of certain pcs can be addressed with pc set analysis, for example, by noting pc invariance among different sets.
Analysts also learn to keep their eyes (ears!) open for certain referential set classes with "tonal" associations. For instance, the diatonic scale (class 7-35 (013568T)), the diatonic octachord (8-23 (0123578T)) and the octatonic scale (8-28 (0134679T)), as well their subsets, are all prominently represented in Stravinsky's music. They have associations with common-practice pitch structures. The ways in which these materials are used sometimes reproduce features of common-practice music, while also projecting differences. For instance, the dominant-seventh-type arpeggios heard at the beginning of Stravinsky's Symphony of Psalms may sound familiar, but analysis suggests that Stravinsky is using these sets of class 4-27 (0258) in an octatonic rather than diatonic context. Conversely, the opening of Petrushka is recognizably diatonic in context, though the set of class 4-23 (0257) Stravinsky uses here is not a common structure in traditional Western tonal music.
One of the most explored aspects of music early 20th-century music is whether or not music that projects pc salience or centricity is "tonal". Tonality has been defined in too many ways to answer this question definitively. Exploring the issue, however, has led to several attempts to combine pc set analysis with other analytical tools, especially with traditional harmonic analysis and with Schenkerian analysis.