Without a doubt, the most useful instruments for Leopold Mozart to learn from, as was for other musicians from that epoch, were the manuscripts of the active composers in Salzburg as well as from abroad, and which were requested by Leopold from his connections in other cities. Maybe we should remember the scores from Antonio Vivaldi's concerts that Johann Sebastian Bach transcribed in order to study them, and it was thanks to those studies that he was able to reach the musical summits of the 6 Brandenburg Concertos? The tendency of obtaining scores from other composers (more or less legally) continued in the case of Leopold, as well as following Wolfgang's compositional formation, keeping up to date with the fashionable styles of the times.
In this epoch, we have many examples of amateur musicians, often of a religious formation, who composed for the necessity of their circle of friends or for work performances. Defining them as amateurs, in some cases, did not impede them from composing in less than an all together pleasurable and fashionable style of the times. The musical simplification that occurred in the transition from Baroque to Galant music rendered compositional musical activity available to more people. To get an idea, just compare the complex polyphonic architecture of Johann Sebastian Bach with the much more simplistic compositions of the musicians of the Salzburg Court, such as Eberlin and Adlgasser.
During his years at the school of Jesuits in Augsburg, Leopold did have instruction in singing, organ and violin with some basic rudiments of musicality (just enough to complete an accompaniment on numbered basses or to create simple harmonic structures for improvisations with simple frequency modulations). In that epoch, orchestra musicians were supposed to know how to play several instruments in order to comply with the various requirements of the sacred and profane. Clearly, though with a few exceptions, such a custom meant that the quality of playing instruments was not always particularly brilliant (an example being Schachtner, a family friend, who was a trumpeter in the Court Orchestra, but who is also described as a violinist and cellist). Leopold, and later his son Wolfgang, played keyboard (harpsichord, and later piano and organ), string instruments (violin and viola) and were able to execute vocal musical compositions. Nannerl's preparation, on the other hand, was focused on keyboard and singing.
In a famous portrait painted in 1763 by Louis de Carmontelle in Paris, we see one of the one of the instrumental formations in which the Mozart "prodigious children" performed during one of their promotional journeys: Wolfgang at the harpsichord, Nannerl singing, Leopold on the violin. The variety of instruments that many performers were meant to know how to play also indicates the inadequate compensation that troubled many of the musicians from that epoch. Many were forced to seek out employment outside of their profession.
Among the friends of the Mozart family, there was, for example, a certain Fink who was a Court trumpeter and organist; to make ends meet, he worked as a vintner at the Ai 3 Mori Inn. Another Salzburg musician, the horn player Ignaz Leutgeb and friend of Wolfgang, who upon returning to Vienna after a falling out with Leopold, asked Wolfgang's father for a loan to open a small shop that sold cheese.
Now let us go back and have a look at Leopold as a composer. His first compositional work was a collection of 6 church and chamber sonatas with three instruments (two violins and a bass), published at his expense in 1740 when he was 21 years old and dedicated to Count Johann Baptist Thurn, President of the Chapter of the Salzburg Cathedral, where he was employed as a Chamberlain and Musician. His subsequent compositional endeavors were two cantatas composed for the Easter season, written respectively in 1741 and 1743 and were quite probably performed at the Princely Court where Leopold Mozart had evidently been appointed by Count Thurn, as well as a scholastic opera entitled "Antiquitas personata" (History personified, or rather, Ancient History up till the Birth of Christ), composed in 1742 and performed at the small student auditorium at the University.
Once hired at the Court, his compositional activity (besides that of executive and educational duties) became legitimate and prolific, enough to enable him to "cover" the civil and religious requirements of the Court, as well as to create music for the Collegium Musicum of Augsburg, where he sent his compositions entitled "Passeggiata in slitta" ("A Sleigh Ride"), "Nozze contadine" ("Country Wedding") and "Sinfonia pastorale" ("Pastorale Symphony"). He composed a relevant number of musical works, many of them masses and church music, pieces for the keyboard, various Symphonies and Divertimenti, concerts and all types of music for festive occasions. Among the interesting facts, we can cite the series of 12 pieces that Leopold Mozart wrote (in collaboration with his friend, Kapellmeister Eberlin) for the pipe organ situated in the fortress that overlooks Salzburg from the hilltop.
Leopold, nevertheless, did not remain mentally closed within the limited confines of the provincial Salzburg. Besides his contacts in his native Augsburg, he cultivated epistolary relations with German musicians and music lovers from Leipzig (Lorenz Mizler) and Berlin (Friedrich Marpurg), as well as with various publishers such as Hulrich Haffner from Nuremburg (who, at his own expense, he had commissioned to print his first published opera, the Six Sonatas Trio dedicated to Count Thurn, and had also published three of his Sonatas for the harpsichord in Italian style) and Gottlieb Immanuel Breitkopf from Leipzig (who inserted numerous compositions of Leopold into his catalogue).
After the birth of Wolfgang, it was true that he dedicated a lot of time to the formation of his prodigious children and the rest of his time to his work related commitments, but he still continued for a long period to compose. He gave up only when he realized, irremediably, that his compositions were outdated and behind the times. And certainly, the production in continual evolution of his son did not contribute to improving his appreciation of his own past work. The fact was, in any case, that Leopold's musical "style" influenced his son, combined with the many other ideas that the young man encountered over time, adopted from other artists and then finally, he came into his own, transforming every note written by him into sublime Mozartian perfection.
The teacher
As a teacher, it is necessary to acknowledge Leopold Mozart's reliable ability which was first acquired in his role as a singing and instrument teacher to children at the Princely Chapel and later, refined in the elaboration of his book, "Violinschule" (A Treaty on the Fundamental Principles of Violin Playing) which was published for the first time in 1756 (the year of Wolfgang's birth) and reprinted in 1769 and again in 1787 (the year of its author's death). The method, published at Leopold's expense by the publisher, Johann Jakob Lotter of Augsburg, had already been widely distributed throughout Europe and was translated into Dutch (1766) and French (1770), as Leopold proudly wrote in his epistolary years later.
Other various editions had been re-proposed over successive historical periods, right up to the present, as its structural and methodological coherence was, and remains, very useful to understanding the execution and expressive techniques of that epoch. The method, in fact, covers every aspect of execution techniques from that era of the transition from Baroque to Classicism and is simple, clear and complete, with relative examples of the positions, use of the bow, phrasing, interpretations of the grace notes, etc. Moreover, as complement to the book and in pointing out the author's desire to be thorough, it has a concise synthesis of the history of music and a basic dissertation on the rules of solfège.