A rock festival, considered synonymous with pop festival, is considered to be a large-scale rock music concert, featuring multiple acts performing an often diverse range of popular music including rock, pop, folk, electronic, and related genres. As originally conceived in the mid to late 1960s, rock festivals were held outdoors, often in open rural areas or open-air sports arenas, fairgrounds and parks, typically lasted two or more days, featured long rosters of musical performers, and attracted very large crowds – sometimes numbering several hundred thousand people.