are made up of, "lines with four stressed beats, separated by other syllables that may vary in number and may include other stressed syllables. The strong beat of the accompaniment coincides with the stressed beats of the verse, and the rapper organizes the rhythms of the intervening syllables to provide variety and surprise". The same technique is also noted in the book How to Rap, where diagrams are used to show how the lyrics line up with the beat – "stressing a syllable on each of the four beats gives the lyrics the same underlying rhythmic pulse as the music and keeps them in rhythm ... other syllables in the song may still be stressed, but the ones that fall in time with the four beats of a bar are the only ones that need to be emphasized in order to keep the lyrics in time with the music". In rap terminology, 16-bars is the amount of time that rappers are generally given to perform a guest verse on another artist's song; one bar is typically equal to four beats of music. History Old school flows were relatively basic and used only for a few syllables per bar, simple rhythmic patterns, and basic rhyming techniques and rhyme schemes. Melle Mel is cited as an MC who epitomizes the old school flow – Kool Moe Dee says, "from 1970 to 1978 we rhymed one way [then] Melle Mel, in 1978, gave us the new cadence we would use from 1978 to 1986". He's the first emcee to explode in a new rhyme cadence, and change the way every emcee rhymed forever. Rakim, The Notorious B.I.G., and Eminem have flipped the flow, but Melle Mel's downbeat on the two, four, kick to snare cadence is still the rhyme foundation all emcees are building on". Artists and critics often credit Rakim with creating the overall shift from the more simplistic old school flows to more complex flows near the beginning of hip hop's new school – Kool Moe Dee says, "any emcee that came after 1986 had to study Rakim just to know what to be able to do. Rakim, in 1986, gave us flow and that was the rhyme style from 1986 to 1994. From that point on, anybody emceeing was forced to focus on their flow". Kool Moe Dee explains that before Rakim, the term 'flow' wasn't widely used – "Rakim