But over the years, Billboard has developed a much more comprehensive system based on sales reports by hundreds of stores and airplay reports from hundreds of radio stations, whose influence is "weighted" according to their ranking in five size categories. Each week, after receiving each report and "weighting" it, Billboard comes up with a sales total and airplay total, which are then added together to determine the final Hot 100.

It's all very complicated, but what it boils down to is that music fans in Chicago can have a significant impact on the national charts, because two of the city's Top 40 radio stations--WLS-AM and WBBM-FM--are among the five stations designated as "platinum," which is the most influential category. (The others are WPLJ-FM and WHTZ-FM in New York, and KIIS-AM/FM in Los Angeles.)

Billboard has separate charts for all those other popular-music formats, which include country, album rock, adult contemporary, dance and black. (Several years ago, with the approval of radio programmers, Billboard replaced its "soul" charts with "black" charts. The title refers not to the race of the artist but to the format of music, which might include anything from pop/dance to R&B/ballads to funk/rock, but would never include hard rock or country. However, many radio stations that contribute to the "black" charts prefer to list their format as "urban.")