In 2010 a number one single usually sold around 100,000 copies per week; sales of around 30,000 were often sufficient to reach the top ten, and a single selling over 6,000 copies could make the top forty. Record companies often release collections of hit singles by various artists as compilation albums, such as the "Now That's What I Call Music!" series. Well-known bands and artists also frequently release collections of their most popular singles as "Greatest hits" albums.