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. In drumming, a fill is defined as a "short break in the groove--a lick that 'fills in the gaps' of the music and/or signals the end of a phrase. It's kind of like a mini-solo. A fill may be played by rock or pop instruments such as the electric lead guitar or bass, organ, or drums, or by other instruments such as strings or horns. In blues or swing-style scat singing, a fill may even be sung. In a hip-hop group, a fill may consist of rhythmic turntable scratching performed by a DJ.