Now Playing: Google's Music Timeline

Beginning in 1950, the timeline starts with jazz, showing which artists ruled the charts and even those one-hit wonders. After jazz, click on pop to see which other subcategories are included, like dance pop. Regardless of the genre you select, the seven most popular artists and albums of that genre sit just below the graph. The key to understanding the most popular genre at a certain time is the thickness of the stripe on the graph: The thicker the stripe, the more popular it was at the time.

However, fans of classical music might be disappointed: Classical music was not included on the timeline. The logic behind this omission is based in the confusion of whether a piece's true release date was when a composer wrote it or when it was recorded by a philharmonic. According to Google, people usually think of classical music in terms of its composition date, not its recording date. Since the music timeline is based on recording date, it didn't make sense to include classical music.

Like Google Sightmap, a heat map showing the most-photographed locations in the world, the music timeline also uses crowd-sourced data from the music on Google Play. This data defines "popular" by the number of users who have a particular artist or album in their music libraries.