Language Log drops the science on the metrics of rap and hiphop, as part of a larger article on Yale University’s recently-published, transcription-error-prone The Anthology of Rap. The above image illustrates the meter of Grandmaster Flash and the Furious Five’s SuperRappin’.
As always in such cases, there’s an interesting question whether this should be thought of as a superficial deviation from an underlyingly square rhythm, or rather as a different draw from a set of available polyrhythmic patterns. For some more discussion, see e.g. “Rock syncopation: Stress shifts or polyrhythms?”, 11/26/2007. Note in any case that the mixture of four-beat and three-beat (lyric) lines evokes the traditional English ballad meter, whatever we’re to make of the variations in alignment.