Of all prosodic variables used to classify languages, rhythm has proved most problematic. Recent attempts to classify languages based on the relative proportion of vowels or obstruents have had some success, but these seem only indirectly related to perceived rhythm. Coupling between nested prosodic units is identified as an additional source of rhythmic patterning in speech, and this coupling is claimed to be gradient and highly variable, dependent on speaker characteristics and text properties. Experimental results which illustrate several degrees of coupling between different prosodic levels are presented, both from previous work within the Speech Cycling paradigm, and from new data. A satisfactory account of speech rhythm will have to take both language-specific phonological properties and utterance-specific coupling among nested production units into account.