The definition of pop music is purposefully flexible as the music that is identified as pop is constantly changing. At any particular point in time it may be easiest to identify pop music as that which is successful on the pop music charts. For the past 50 years the most successful musical styles on the pop charts have continually changed and evolved. However, there are some consistent patterns in what is identified as pop music.
Since the mid-1950's pop music has usually been identified as the music and the musical styles that are accessible to the widest audience. This means the music that sells the most copies, draws the largest concert audiences, and is played most often on the radio. After Bill Haley's "Rock Around the Clock" hit #1 on music charts in 1955 the most popular music became the records influenced by rock 'n roll instead of the songs and light standards that had dominated TV's Your Hit Parade weekly countdown show. Since 1955 the music that appeals to the widest audience, or pop music, has been dominated by sounds that are still rooted in basic elements of rock 'n roll. .