The term "rock and roll" now has at least two different meanings, both in common usage. The American Heritage Dictionary and the Merriam-Webster Dictionary both define rock and roll as synonymous with rock music. Allwords.com, however, refers specifically to the music of the 1950s. For the purpose of differentiation, this article uses the latter definition, while the broader musical genre is discussed in the rock music article.
In the earliest rock and roll styles of the late 1940s and early 1950s, either the piano or saxophone was often the lead instrument, but these were generally replaced or supplemented by guitar in the middle to late 1950s. The beat is essentially a boogie woogie blues rhythm with an accentuated backbeat, the latter almost always provided by a snare drum. Classic rock and roll is usually played with one or two electric guitars (one lead, one rhythm), a string bass or (after the mid-1950s) an electric bass guitar, and a drum kit.
Rock and roll began achieving wide popularity in the 1960s. The massive popularity and eventual worldwide view of rock and roll gave it a widespread social impact. Bobby Gillespie writes that "When Chuck Berry sang 'Hail, hail, rock and roll, deliver me from the days of old,' that's exactly what the music was doing. Chuck Berry started the global psychic jailbreak that is rock'n'roll."
Far beyond simply a musical style, rock and roll, as seen in movies and on television, influenced lifestyles, fashion, attitudes, and language. It went on to spawn various sub-genres, often without the initially characteristic backbeat, that are now more commonly called simply "rock music" or "rock."