In popular music, a fill is a short musical passage, riff, or rhythmic sound which helps to sustain the listener's attention during a break between the phrases of a melody. "The terms riff and fill are sometimes used interchangeably by musicians, but [while] the term riff usually refers to an exact musical phrase repeated throughout a song", a fill is an improvised phrase played during a section where nothing else is happening in the music. While riffs are repeated, fills tend to be varied over the course of a song. For example, a drummer may fill in the end of one phrase with a sixteenth note hi-hat pattern, and then fill in the end of the next phrase with a snare drum figure.