Alternative Airplay is a music chart published in the American magazine Billboard since September 10, 1988. It ranks the 40 most-played songs on alternative and modern rock radio stations. Introduced as Modern Rock Tracks, the chart served as a companion to the Mainstream Rock chart, and its creation was prompted by the explosion of alternative music on American radio in the late 1980s. During the first several years of the chart, it regularly featured music that did not receive commercial radio airplay anywhere but on a few modern rock and college rock radio stations. This included many electronic and post-punk artists. Gradually, as alternative rock became more mainstream, alternative and mainstream rock radio stations began playing many of the same songs. By the late 2000s, the genres became more fully differentiated with only limited crossover. The Alternative Airplay chart features more alternative rock, indie pop, and pop punk artists while the Mainstream Rock chart leans towards more guitar-tinged blues rock, hard rock, and heavy metal.