Both American Top 10 and American Top 20 can trace their history to "Casey's Countdown" and "Casey's Hot 20" respectively. Both Casey shows were spin-offs of "Casey's Top 40", the competitor to AT40 created by Casey Kasem when he left the original AT40 back in 1988.
As pop (CHR) radio was declining, it was decided that there had to be another way to get more stations to carry a Casey Kasem countdown show. Since many stations were turning to the Adult Contemporary format, a new Casey show was created, "Casey's Countdown". Casey's Countdown debuted in 1992 and counted down the top 25 songs on Radio and Records' AC chart. The show was successful and was a good compliment to the pop CT40.
By 1994, radio had continued to splinter. A new format was arising, Hot Adult Contemporary aka Adult Top 40. These stations played 'harder' songs that AC would not play, but these stations also didn't play the rap and hip-hop found on some CHR stations. They needed their own countdown show. In April 1994 Radio and Records debuted its Hot AC chart, and in November 1994 "Casey's Hot 20" was born. The show counted down the top 20 of the Hot AC chart. Casey's Countdown continued to use the AC chart but started only counting down the top 20 vs. the top 25.
Both shows were three hours and played extras, former number ones, and "Requests and Dedications".
The shows continued until Casey left Westwood One in 1998 to restart AT40 with AMFM Radio Networks. The shows were both renamed "American Top 20", but the shows essentially remained the same. The shows still play the extras, and the shows are now the only ones with "Long Distance Dedications".
Both AT20s used Radio & Records' charts for much of the period from 1998 to early 2003. However, there was a period in 2000-2001 where the shows used an unpublished Mediabase 24/7 chart. Songs were removed from that chart based on an unknown recurrent rule, and sometimes songs would seem to be "purged" from the chart to allow new songs to enter. In early 2003, AT20 AC again switched to an unpublished Mediabase 24/7 chart. This time some specifics were known about the chart. The chart is based on audience impressions rather than the number of times a song is played, and songs played during the syndicated "Delilah" show are not counted toward the chart. Songs seem to be removed from this chart at about the same time as they are on Radio and Records' AC chart.
During the March 20-21, 2004 weekend, American Top 20 AC became American Top 10 AC. The show now only plays the top 10 of the unpublished chart each week. In addition to the regular extras played on the show, there are "themed" extras, known as the AT10 Spotlight. For example, for the first AT10, the spotlight was "Band Members Gone Solo". The jingles were changed as well. In 2005, the show once again used the R&R AC chart.
New features added to AT20 and AT10 in 2004 and 2005 are the Book of Records and "Whatever Happened to...".
In the fall of 2005, a special edition of AT10 began airing in New York City. In this special countdown, done only for WLTW, Casey just counts down the top 10 AC songs. Thus, the show only lasts one hour in length.
In August of 2006, Radio and Records quit using Mediabase 24/7 data as a source for its charts. Thus, with the airings of August 19, the shows began using charts that were not published in a radio trade magazine.
In mid 2009, Casey and Premiere Radio Networks mutually agreed to end both AT20 and AT10. Part of Casey's decision was so he could write his memoirs. The final shows aired on the weekend of July 4, 2009. This was the 39th anniversary of Casey starting American Top 40.
During the 11+ years that AT20 Hot AC was on the air, there were 68 number one songs. The first #1 on AT20 was "3 AM" by Matchbox Twenty. The final #1 was "Second Chance" by Shinedown.