Wolfgang Amadeus Mozart began his series of preserved piano concertos with four that he wrote at the age of 11, in Salzburg: K. 37 and 39-41. The autographs, all held by the Jagiellonian Library, Kraków, are dated by his father as having been completed in April (K. 37) and July (K. 39-41) of 1767. Although these works were long considered to be original, they are now known to be orchestrations of sonatas by various German virtuosi. The works on which the concertos are based were largely published in Paris, and presumably Mozart and his family became acquainted with them or their composers during their visit to Paris in 1763–64. By using movements from the sonatas of other composers, the young Mozart seems to have begun to learn how to cope with the structural problems of composing in the piano concerto form. Indeed, it may be that Leopold Mozart had devised this as a compositional teaching method. If so, it seems that this may have been the first time this had been done by composer. This is perhaps supported by two facts: First, Leopold excluded the first four concertos from his 1768 list, suggesting that he may not have considered them true compositions by his son. Second, the autographs of the four works are the joint products of both Mozart and Leopold (although K. 41 is mainly in Leopold's hand alone).