From its creation in July 1970 through late November 1991, AT40 used the Hot 100 chart to get its countdown. When Billboard changed the Hot 100 in November 1991, AT40 switched charts to better reflect the music heard on its affiliates. This chart was the "Top 40 Radio Monitor" chart, whose name later changed to "Hot 100 Airplay". However, this also did not sit well with all affiliates, and AT40 finally switched to the "Top 40 / Mainstream" chart. During the show's original run, 5363 songs made the top 40, and there were 552 Number Ones. The first number one song was "Mama Told Me Not to Come" by Three Dog Night, and the final number one song was "On Bended Knee" by Boyz II Men. Below are samples of some important charts in Classic AT40 history.
The last countdown to be based on the Hot 100 used this chart: (Whole chart on page 54 of this document)
The first countdown to use the airplay only data: (Whole chart here)
The final AT40 chart, which aired in January 1995: (Whole chart here)
Please note that all charts shown above are copyrighted by Billboard/BPI communications, Inc. Reproduced here without permission, but all trademark names and copyrights are the property of their respective holders and are here for descriptive/informative purposes only. *According to American Radio History website, Billboard released its old magainzes into the public domain, but this cannot be confirmed at this time.
The last AT40 with Casey used the Radio and Records charts, except for a period from October 2000 - August 2001 . Here is an excerpt from an example of that chart, the CHR (Pop) chart:
From October 2000 through August 2001, AT40 used an unpublished chart from Mediabase 24/7.
The AT40 with Ryan Seacrest (2004-) uses a special chart compiled by Mediabase 24/7.
Please note that the above example was copyrighted by Radio and Records Magazine. Reproduced here without permission, but all trademark names and copyrights are the property of their respective holders and are here for descriptive/informative purposes only.