The London Sketchbook (German: Londoner Skizzenbuch), K.15 a-ss (Anh. 109b) is a series of 43 untitled pieces and sketches written by Wolfgang Amadeus Mozart between 1764 and 1765 while in London (see the Mozart family's grand tour). The set of works is denoted by its K6 number, followed by its respective letter, i.e. 15a, 15b, 15c, etc.
Most pieces are extremely short, normally lasting from 40 seconds to a minute; however, some span as long as four minutes in total (see K. 15t). According to the Neue Mozart-Ausgabe, the intended purpose of this book was not for musical exercise, as once thought, rather for the young Mozart, who had just learned how to use pen and ink, to write down his own inspiration without needing anyone's help. Corrections by his father Leopold appear in pencil only.