Pop music is a genre of popular music that originated in its modern form in the United States and United Kingdom during the mid-1950s.[4] The terms "popular music" and "pop music" are often used interchangeably, although the former describes all music that is popular and includes many different styles. "Pop" and "rock" were roughly synonymous terms until the late 1960s, when they became increasingly differentiated from each other. Although much of the music that appears on record charts is seen as pop music, the genre is distinguished from chart music. Pop music is eclectic, and often borrows elements from other styles such as urban, dance, rock, Latin, and country; nonetheless, there are core elements that define pop music. Identifying factors include generally short to medium-length songs written in a basic format (often the verse-chorus structure), as well as common use of repeated choruses, melodic tunes, and hooks.