Rock music is a broad genre of popular music that originated as "rock and roll" in the United States in the late 1940s and early 1950s, developing into a range of different styles in the mid-1960s and later, particularly in the United States and United Kingdom. It has its roots in 1940s and 1950s rock and roll, a style that drew directly from the blues and rhythm and blues genres of African-American music and from country music. Rock also drew strongly from a number of other genres such as electric blues and folk, and incorporated influences from jazz, classical, and other musical styles. For instrumentation, rock has centered on the electric guitar, usually as part of a rock group with electric bass guitar, drums, and one or more singers. Usually, rock is song-based music with a time signature using a verse–chorus form, but the genre has become extremely diverse. Like pop music, lyrics often stress romantic love but also address a wide variety of other themes that are frequently social or political. Rock was the most popular genre of music in the United States and much of the Western world from the 1950s to the 2010s. Rock musicians in the mid-1960s began to advance the album ahead of the single as the dominant form of recorded music expression and consumption, with the Beatles at the forefront of this development. Their contributions lent the genre a cultural legitimacy in the mainstream and initiated a rock-informed album era in the music industry for the next several decades. By the late 1960s "classic rock" period, a number of distinct rock music subgenres had emerged, including hybrids like blues rock, folk rock, country rock, southern rock, raga rock, and jazz rock, many of which contributed to the development of psychedelic rock, which was influenced by the countercultural psychedelic and hippie scene. New genres that emerged included progressive rock, which extended the artistic elements, glam rock, which highlighted showmanship and visual style, and the diverse and enduring subgenre of heavy metal, which emphasized volume, power, and speed. In the second half of the 1970s, punk rock reacted by producing stripped-down, energetic social and political critiques. Punk was an influence in the 1980s on new wave, post-punk and eventually alternative rock. From the 1990s, alternative rock began to dominate rock music and break into the mainstream in the form of grunge, Britpop, and indie rock. Further