Mainstream Top 40

Mainstream Top 40 (also called Pop Songs on billboard.com and sometimes referred to as Top 40/CHR) is a 40-song music chart published weekly by Billboard Magazine which ranks the most popular songs being played on a panel of Top 40 radio stations in the United States. The rankings are based on radio airplay detections as measured by Nielsen Broadcast Data Systems (Nielsen BDS). Arbitron refers to the format as contemporary hit radio (CHR).

The chart debuted in Billboard Magazine in its issued date October 3, 1992, with the introduction of two Top 40 airplay charts, Mainstream and Rhythm-Crossover. Both Top 40 charts measured "actual monitored airplay" from data compiled by Broadcast Data Systems (BDS). The Top 40/Mainstream chart was compiled from airplay on radio stations playing a wide variety of music, while the Top 40/Rhythm-Crossover chart was made up from airplay on stations playing more dance and R&B music. Both charts were "born of then-new BDS electronic monitoring technology" as a more objective and precise way of measuring airplay on radio stations. This data was also used as the airplay component for Hot 100 tabulations.

Top 40/Mainstream was published in the print edition of Billboard from its debut in October 1992 through May 1995, when both Top 40 charts were moved exclusively to Airplay Monitor, a secondary chart publication by Billboard. They returned to the print edition in the August 2, 2003, issue.

Songs on the chart are ranked by the total number of spins detected per week. Songs which gain plays or remain flat from the previous week will receive a bullet. A song will also receive a bullet if its percentage loss in plays does not exceed the percentage of monitored station downtime for the format. If two songs are tied in total plays, the song with the larger increase in plays is placed first.

Since the introduction of the chart until 2005, songs below No. 20 were moved to recurrent after 26 weeks on the chart. In the chart week of December 3, 2005, songs below No. 20 were moved to recurrent after 20 weeks on the chart. Since the issue dated December 4, 2010, songs older than 20 weeks on the chart are moved to recurrent after they drop below No. 15.

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