In music, minimalism is a way of composing classical music which became popular during the second half of the 20th century. It is a way of composing by using a simple idea which is repeated many times. A minimalist piece of music usually takes a short idea which may be a melodic or a rhythmic pattern. This idea is then repeated many times, but each time it changes slightly. Sometimes this may be done by two or more instruments which start off by playing their notes or chords together but, as one is going slightly faster than the other, they gradually become “out of sync” (not synchronized, i.e. not together), then eventually they catch up with one another again. Usually this music is tonal so that it is firmly in one key. "A steady pulse is kept throughout the whole piece." 