Pop music (a term that originally derives from an abbreviation of "popular") is a genre of popular music, which originated in its modern form in the Western world during the 1950s and 1960s, deriving from rock and roll. The terms "popular music" and "pop music" are often used interchangeably, although the former is a description of music which is popular (and can include any style). As a genre, pop music is extremely eclectic, often borrowing elements from other styles including urban, dance, rock, Latin and country; nonetheless, there are core elements which define pop. Such elements include generally short to medium-length songs, written in a basic format (often the verse-chorus structure), as well as the common employment of repeated choruses, melodic tunes, and hooks. So-called "pure pop" music, such as power pop, which has been described as mixture of hard rock and melodic pop music and tends to be more aggressive than pop rock, features all these elements, using electric guitars, drums and bass for instrumentation.