AT40 used the top 40 songs from the "Billboard" Hot 100 singles chart from the show's inception in 1970 to November 23, 1991. The chart was widely regarded as the industry standard for tracking the popularity of singles, and was thus a natural choice to be used. Kasem would frequently announce during the show that Billboard was the only source for the countdown. While using these charts worked well for the first half of the 1970s, as music changed during the decade and disco became popular on the charts, some rock stations began to drop the show because of complaints from program directors that AT40 was playing too many songs not normally heard on their stations.