An equaliser divides the audio spectrum into bands that can be turned up or down. On a parametric EQ, each band has three main controls: the centre frequency, the amount of boost or cut, and the bandwidth or Q, which sets how many neighbouring frequencies are affected. Most equalisers also include high-pass and low-pass filters, which cut one end of the spectrum more steeply than an ordinary EQ band.
EQ is used both to correct problems, such as a boomy piano or a harsh cymbal, and to shape character. In jazz lo-fi it often helps sampled and played parts sit together: trimming low end from a sampled loop so the kick and bass can breathe, or gently dipping the upper treble to give the whole track a warm, rounded tone.