Q:Given the following passage  "Chopin took the new salon genre of the nocturne, invented by the Irish composer John Field, to a deeper level of sophistication. He was the first to write ballades and scherzi as individual concert pieces. He essentially established a new genre with his own set of free-standing preludes (Op. 28, published 1839). He exploited the poetic potential of the concept of the concert étude, already being developed in the 1820s and 1830s by Liszt, Clementi and Moscheles, in his two sets of studies (Op. 10 published in 1833, Op. 25 in 1837).",  answer the following question. Note that the answer is present within the text.  Question: What set Chopin's preludes apart?
A:
free-standing