During the mid-to-late-1990s, the "Mix" format—a loosely defined mixture of Top-40 and classic rock with something of an emphasis on adult contemporary music—began to appear across the country. While the format has no particular standard identity, most "mix" stations have rotations consisting largely of pop and rock music from the 1980s and 1990s (and often the 1970s), with some current material mixed in. In addition, stations devoted to the pop music of the 1970s, 1980s, and 1990s on their own have developed as the audiences that grew up with that music grew older and nostalgic for the sounds of their youth. The continued play of such songs on such stations has also exposed the music to younger generations that can take a renewed interest in the music, prolonging its shelf life beyond what conventional wisdom in the industry may suggest.