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. Often, this involves aligning the beats of the music sources so their rhythms do not clash when they are played together. DJ equipment also allows the DJ to preview a source of recorded music in headphones before playing it for the audience. Previewing in headphones helps the DJ pick the next track they want to play and seek within the track to a time when it will mix well with the currently playing music.