In the Classical period, not many composers wrote preludes. More preludes were composed in the 19th century (the period of Romanticism). Frédéric Chopin wrote a collection of twenty-four short piano pieces which he called “Preludes”. Just like Bach's "Well-Tempered Clavier", there is a prelude in every major and minor key signature, but unlike Bach's, these pieces are not meant to be followed by anything; they are just separate pieces of music that are often performed together. Some are not too difficult to play, but others are very hard (virtuoso).