A contrasting piece would depend on what it's contrasting. It just means something else that is different from another.
Contrasting pieces at the composer level would be Beethoven's piano sonata #1 and #32. Both piece are strikingly different.
Contrasting pieces occur among composers, Liszt's works compared to Chopin's.
Contrasting pieces occur over periods: Baroque, Classical, Romantic, Impressionism, 20th Century, Modern.
In your case, it most likely means playing a contrasting piece for the Bach piece, which is from the Baroque era, against another piece from the other eras. You could use a sonata by Mozart, Haydn, or Beethoven, which are usually requirements for most music program auditions.
For the audition pieces I am going to do, I will have to perform 3 or 4 contrasting works. Here is an example of the contrasting pieces I am tentatively planning on:
Bach Well Tempered Klavier Book II, prelude & fugue in Fmoll (baroque)
Beethoven piano sonata Op10 No2 (classical)
Alkan Esquisses Op 63 No5 (romantic)
Rachmaninov Prelude in C#moll (rachmaninov

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