The earliest "Raps" were usually spoken by the DJ, as the concept of an MC had not really developed. Herc stated in an interview with Davey D, that they began as announcements, "Last Call at the Bar" or "Owner of a blue van, please move it." But DJs began to use these annoucements to further hype the crowd, in the style of the "Toasters" from Jamaican Dub music. They began to create short, simple chants or call-and-answer patterns with the crowd. DJ Kool Herc, Melle Mel and Bizee Bee are all expert practitioners of this uncluttered, classic style. They usually contain one syllable per beat, and feature a rhyme at the end of the line only. It was fairly common to ape or parody nursery rhymes or schoolyard songs, due to the similar structure.
As Hip-Hop grew and developed into the mid-1980s, the next generation of rappers developed the newborn vocal style by modulating the pacing from one line to the next. Kyle Adam's essay "Music Theory Online: On The Metrical Techniques of Flow In Rap Music" contrasts 80s rappers with their '90s counterparts. Instead of a repeating, staccato pattern, legendary MCs such as Rakim, Chuck D, Kool Moe Dee and KRS-One let variations in the music lead the rhythm of their raps speeding or slowing lines according to the beat and lyrical content, drawing on the free-form traditions of be-bop and blue-note to bring a deeper, poetic dimension to the genre. The intangible concept of "Flow" is one of the most sought-after characteristics of a rapper. A way to study and understand flow is to listen to some foreign-language rappers. If you don't understand the lyrics, you are forced to concentrate on the pacing and dynamics of the vocal.
Though rappers began to vary the rhythm of their lines, the rhyming syllables still only ever came at the end of the lines, in the standard "Limerick" rhyme scheme. In the latter half of the 1980s and the early 1990s, rappers began to stack rhyming syllables within each line. Rakim, Big Daddy Kane and Big Punisher are some notable artists who "fold" rhyming syllables into a line. Creating rhyme points, dividing the line into sub-lines. A good example of this appears in Big Punisher's song "Twinz." The four line break consists of the words "When in the middle of Little Italy, little did we know that we riddled for middlemen who didn't do diddly." This creates a cascade of rhyming words that don't seem to fall at any uniform place in the line, creating the sensation of a loop that gets shorter with each repetition, accelerating the listener toward the end of the verse.
Hip-Hop music generally falls between 85 and 100 beats per minute. It's not a particularly fast genre of music. But with most musical pursuits, the ability to perform anything at very high speed, reflects a degree of dedication and skill. Big Daddy Kane is credited with being the creator of the "Fast-Rap" of rhyming, often placing syllables in double, or even quadruple-time. This has been carried on by MCs such as Twista, Tec-9 and Busta Rhymes, with each of them at one time holding the Guiness World Record for "most syllables per minute."
Much of the subject matter featured in Raps is often used to discredit or demonize the entire Hip-Hop genre. The topics of crime, drugs and violence are commonly the focus of raps, as these things unfortunately fill the lives of many acclaimed artists. There are others who buck this trend, filling their music with socio-political tracts, dense, extended metaphors, or complex surreal imagery, usually in an attempt to provoke furhter thought from the listener. KRS-One, is largely considered to be the first "Conscious Rapper," but many have followed in his footsteps. Lupe Fiasco, GZA, Del Tha Funkee Homosapien and Aesop Rock have all been commended for their dense, unconventional lyricism.