The Berlin museum possesses an early clarinet with two keys, marked J.B. Oberlender, derived from the Snoeck collection. Paul de Wit's collection has a similar specimen by Enkelmer. The Brussels Conservatoire possesses clarinets with two keys by Flemish makers, G.A. Rottenburgh and J.B. Willems[27]; the latter, with a small bulb and bell, is in G a fifth above the C clarinet. The next improvements in the clarinet, made in 1720, are due to J. Denner, probably a son of J.C. Denner. They consisted in the addition of a bell and in the removal of the speaker-hole and key nearer the mouthpiece, involving the reduction of the diameter of the hole. The effect of this change of position was to turn the B[natural] into B flat, for J. Denner introduced into the hole, nearly as far as the axis of the bore, a small metal drainage tube[28] for the moisture of the breath. In the modern clarinet, the same result is attained by raising this little tube slightly above the surface of the main tube, placing a key on the top of it, and bending the lever. In order to produce the missing B[natural], J. Denner lengthened the tube and pierced another hole, the low E, covered by an open key with a long lever which, when closed, gives the desired B as its twelfth, thus forming a connexion between the two registers. A clarinet with three keys, of similar construction (about 1750), marked J.W. Kenigsperger, is preserved in the Bavarian national museum, at Munich. Another in B flat marked Lindner[29] belongs to the collection at Brussels. About the middle of the 18th century, the number of keys was raised to five, some say[30] by Barthold Fritz of Brunswick (1697-1766), who added keys for C# and D#. [Illustration] According to Altenburg[31] the E flat or D# key is due to the virtuoso Joseph Beer (1744-1811). The sixth key was added about 1790 by the celebrated French virtuoso Xavier Lefébure (or Lefèvre), and produced G#. [Illustration] Anton Stadler and his brother, both clarinettists in the Vienna court orchestra and instrument-makers, are said to have lengthened the tube of the B flat clarinet, extending the compass down to C (real sound B flat). It was for the Stadler brothers that Mozart wrote his quintet for strings, with a fine obbligato for the clarinet in A (1789), and the clarinet concerto with orchestra in 1791.