DJs use equipment that can play at least two sources of recorded music simultaneously and mix them together. This allows the DJ to create seamless transitions between recordings and develop unique mixes of songs. Often, this involves aligning the beats of the music sources so their rhythms do not clash when they are played together, either so two records can be played at the same time, or to enable the DJ to make a smooth transition from one song to another. An important tool for DJs is the specialized DJ mixer, a small audio mixer with a crossfader and cue functions. The crossfader enables the DJ to blend or transition from one song to another. The cue knobs or switches allow the DJ to pre-listen to a source of recorded music in headphones before playing it for the live club or broadcast audience. Previewing the music in headphones helps the DJ pick the next track they want to play, cue up the track to the desired starting location, and align the two tracks’ beats in traditional situations where auto sync technology is not being used. This process ensures that the selected song will mix well with the currently playing music. DJs may also use a microphone to speak to the audience; effects units such as reverb to create sound effects; and electronic musical instruments such as drum machines and synthesizers.