Rock
Rock & Roll is often used as a generic term, but its sound is rarely predictable. From the outset, when the early rockers merged country and blues, rock has been defined by its energy, rebellion and catchy hooks, but as the genre aged, it began to shed those very characteristics, placing equal emphasis on craftmanship and pushing the boundaries of the music. As a result, everything from Chuck Berry's pounding, three-chord rockers and the sweet harmonies of the Beatles to the soulful pleas of Otis Redding and the jarring, atonal white noise of Sonic Youth has been categorized as "rock." That's accurate — rock & roll had a specific sound and image for only a handful of years. For most of its life, rock has been fragmented, spinning off new styles and variations every few years, from Brill Building Pop and heavy metal to dance-pop and grunge. And that's only natural for a genre that began its life as a fusion of styles.

Country
Country music is about tradition, yet its simple form lends itself to endless variations on similar themes. Like blues — the two genres often shared themes, melodies and songs — country is a simple music at its core. Most of its songs are built around three chords and a plain melody, but these forms are so basic, they allow for many different styles, from the gritty sounds of honky tonk to the jazzy improvisations of Western Swing. Country music grew out of American Southern folk music, both Appalachian and blues, and old-time country was simple and folky, with just guitars and fiddles. As the genre progressed, old time music evolved into the rhythmic guitar-and-fiddle driven traditional country that became the foundation of modern country music, from honky tonk and Western Swing to the pop-oriented Countrypolitan and rock-inflected Bakersfield Sound.

Jazz
Jazz has been called America's classical music, and for good reason. Along with the blues, its forefather, it is one of the first truly indigenous musics to develop in America, yet its unpredictable, risky ventures into improvisation gave it critical cache with scholars that the blues lacked. At the outset, jazz was dance music, performed by swinging big bands. Soon, the dance elements faded into the background and improvisation became the key element of the music.