A music chart is a ranking of popular music songs or albums. Examples are record charts but today there are also mp3 charts and also not specific for any medium. The main criteria for the ranking are the sales of records or CDs. Some charts are specific to a particular musical genre and most to a particular geographical location. The most common period of time covered by a chart is one week, with the chart being printed or broadcast at the end of this time. Summary charts for years and decades are then calculated from their component weekly charts. Component charts have become an increasingly important way to measure the commercial success of individual songs.