The clarinet was the last instrument of the woodwind family to emerge as a modern orchestral instrument. In Mozart's day in the late 18th century, it was still not universally used. It is found in only a handful of Haydn's symphonies, and even Mozart, who loved its sound so much, included it in only a few of his scores.

In those days, the clarinet was undergoing constant changes from the early 18thcentury instrument, which had only two keys, to the one with five keys that became standard around 1760. The orchestra of Mannheim, which Mozart visited in 1778, was one of the first to incorporate clarinets on a regular basis. In one word, the clarinet was still something of a novelty, and Mozart exclaimed in one of his letters to his father after his trip to Mannheim: "Alas, if only we also had clarinets [in Salzburg]."

A decade later in Vienna, Mozart did have clarinets at his disposal. He had become friends with the virtuoso Anton Stadler, whose brother, Johann, was also a clarinet player. Anton Stadler had participated in performances of Mozart's works since at least 1784, and later inspired two of the composer's most magnificent late masterpieces, the Quintet in A major for clarinet and string quartet (K. 581) and the present concerto.

We must know that the compass of the clarinet is divided into registers that greatly differ in character and timbre. The low register, the so-called "chalumeau," is one of the clarinet's most wonderful features, and Stadler, together with Theodor Lotz, Royal Instrument Maker to the Viennese court, experimented with its extension. Their experiments resulted in a clarinet that could go a major third below the regular instrument. Stadler called this a "bass clarinet," but we today call it a "basset" (i.e. "little bass"), to avoid confusion with the modern bass clarinet, which is an octave lower.

It has long been known that both the quintet and the concerto were written for this extended clarinet. The principal evidence comes from a review of the concerto's first edition, published in 1802 in the Allgemeine Musikalische Zeitung in Leipzig. In it, the anonymous reviewer showed how the solo part had been changed by the editors to become accessible to players of the regular clarinet; he pointed out that extended instruments were extremely hard to come by. In a prophetic statement that foreshadowed 20th-century editorial methods, the reviewer concluded: "Thanks are due to the editors for these transpositions and alterations, although they have not improved the concerto. Perhaps it would have been better to publish it in the original version and insert these transpositions and alterations in smaller notes."

Unfortunately, this advice cannot be followed as the original manuscript of the concerto is lost. Nor have any 18th-century basset clarinets survived; it may well have been an instrument that no one but Stadler played even then. Therefore, both the music and the instrument had to be reconstructed before Mozart's orig