In 1786, the year in which he completed his best-known Horn Concerto (K.495 in E-flat major), Mozart began work on another Concerto for Horn and Orchestra in E major. Only a fragment of the first movement survives. An element of mystery surrounds the 91-bar incomplete manuscript which Mozart left. Why was the work left unfinished? Why the highly unusual key of E major? Who was the intended soloist? The recipient of the concerto was most likely to have been the Viennese cheesemonger and hornplayer Leutgeb – a friend and also the butt of Mozart’s jokes. However, there is no confirmation for this theory and there is a suggestion that Leutgeb’s lips may have been starting to struggle with the technical demands which Mozart made, and looked set to stretch in K.494a. The 91 bars which Mozart wrote contain all of the sparkle, beauty and character to be expected of one of his wind concertos. However, they also hint at something more substantial, arguably pointing the way to the Clarinet Concerto of 1791, an undisputed masterpiece.
Making a realisation of the 91 bars into a full first movement of 254 bars involves less actual new composition than might be assumed. The main ideas for the movement exist in the opening orchestral exposition. The formal doctrines of the Classical period would have made wide divergence from these ideas an impossibility. This was particularly so on the horn, which was a somewhat limited instrument at that time since it had no valves. The process of making the realisation can be seen as somewhat like adding the background to a painting for which the foreground and main characters are mostly complete.