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).