The Klavierstück in F (Piece in F for keyboard), K. 33B, is a keyboard work written by Wolfgang Amadeus Mozart sometime during early October of 1766; its tempo marking is Allegro. It is 26 bars long, divided into two sections of 12 and 14 bars, each repeated.
Mozart wrote this piece in pencil on the back of a circular by the Zürcher Musikkollegium (Zürich Music College) dated 30 September 1766, when the Mozart family came to the end of their Grand Tour. The circular invited sponsors, music lovers and other people who might be interested to concerts by "the young (9 years old) Master Mozart as well as his maiden sister" (Maria Anna – Nannerl) on October 7 and 9. It can be assumed that Mozart played or improvised this lively piece in one of those concerts, the programs to which are lost. Mozart wrote it on the back of the circular probably as a souvenir for the College's board; otherwise he would have used his sister's notebook or his third book of sketches (Skizzenbuch), which is also lost. The autograph became known very late (in 1942), and so is missing from Alfred Einstein's 3rd edition of the Köchel catalogue.
The piece appeared in the 1984 film Amadeus when the child Mozart played it blindfolded on the harpsichord and then blindfolded on the violin.