the mainstream top 40 is an airplay chart from "billboard" magazine. it is also known as pop songs on billboard.com. people often mistook it for the now discontinued pop 100 airplay chart. the chart is solely based on radio airplay, ranking the week's hottest pop songs based on monitored radio detections from a panel of 200 top 40/chr radio stations in the united states as measured by nielsen bds nielsen broadcast data systems. arbitron sometimes refers to the format as contemporary hit radio.