Ultratop 40 Singles, often called Ultratop 40, is the weekly chart of best-selling singles in Wallonia, Belgium. Its equivalent covering the Flanders region is Ultratop 50. Both charts are made and published by the Ultratop organization. Like the UK Singles Chart, Ultratop creates charts based on retail record sales of around 500 retail outlets and legal digital downloads. Currently GfK is the market observer of the charts. The chart broadcasts on Radio Contact on Saturdays from 12:00 to 14:00. The combined number of Ultratop chart listeners on radio or TV stations exceeds two million each week. To celebrate the 10th anniversary of the charts in 2005 a jubilee book was published. It covered all the 15,282 singles from 5,882 artists thus far.