Longer works are often divided into self-contained pieces, called movements, often with contrasting characters or moods. For instance, symphonies written during the Classical period are usually divided into four movements: (1) an opening Allegro in sonata form, (2) a slow movement, (3) a minuet or scherzo, and (4) a final Allegro. These movements can then be further broken down into a hierarchy of smaller units: first sections, then periods, and finally phrases.