The opposite situation is what Krebs calls metrical dissonance.I will argue that in jazz styles where swing rhythm is pervasive, halvingthe prevailing note-value can produce metrical dissonance, and that this dissonance is one of the main factors contributing to listeners’ perception of double time.To consider the idea of swing eighth notes in terms of Krebs’ metrical layers, I will callthe quarter-note level of rhythm the “beat layer” and the level at which quarter notes have been subdivided into swing eighths the “sub-beat layer.” The beat layer and sub-beat layer can be regarded as two different ways of grouping a third layer of

Music theorists have long been aware that different levels of beat are often simultaneously available to the listener. Harald Krebs (1999) uses the term “metrical layers” to describe this phenomenon. In the normal case, metrical layers nest neatly insideone another; they are metrically consonant with each other. This occurs, for example, in Pachelbel’s Canonas the prevailing note-values are repeatedly halved. Figure 3.1 shows this nesting relationship between layers. There is no need for a separate micropulse layer because the layers are metrically consonant: both eighth notes and quarter notes can be expressed as groupings of sixteenth notes.

“Double time”is defined by the New Grove Dictionary of Jazzas “the apparent doubling of the tempo [...] achieved by halving the prevailing note-value” (Kernfeld, 2002). In an explanation of the same term written for a popular audience, the book What to Listen for in Jazzclarifies that double time “involves a doubling of tempo in the rhythm section, a doubling of the general speed of the melody line, or both” (Kernfeld, 1997).The Grove’s use of the word “apparent” implies that the tempo has not in fact doubled. To emphasize this point, some authors prefer the more precise term “double-time feel,” reserving the term “double time” for a true doublingof tempo(Levine, 1995).