Until the mid-18th century, most clarinets only had two keys. The range of the instrument extended from F3 to G4 through the tone holes only. Pressing the upper key could produce a4, and pressing both the upper and lower keys could produce B4. By overblowing, the notes C5 to G6 could also be played. However, B4 was rarely used due to poor intonation. Therefore, a third key was later added to the instrument, which also changed the lowest note to E3. 
There were numerous problems with the 18th century instrument. The tone quality of the forked notes differed from the others, and the intonation of certain notes was not good. This had several reasons.
This has made it extremely difficult to create a clarinet with perfect intonation (even to this day).
Instrument makers produced differently pitched instruments to circumvent the intonation difficulties by exchanging them. Therefore, in addition to the earlier instruments pitched in D and C, G-, B-, A-, and Bb-clarinets also appeared. However, this did not solve the problem, because in addition to the intonation not being improved on individual instruments, musicians in orchestras had to buy several different instruments and adapt to their characteristics. Frequent instrument changes caused difficulties with intonation due to the cooled instrument bodies. In addition, the change was inconvenient due to the different sizes of mouthpieces. Nevertheless, I believe it is important to mention that musicians and composers appreciated the differently pitched clarinets because they could create completely different timbres with them. Carl Baermann writes as follows in his book "Vollständige Clarinett Schule" about this phenomenon:
"This is the period in which Iwan Müller made his restless attempts and wanted to establish a normal B-clarinet on which one should be able to play with ease in all keys, thus avoiding the annoying change between A- and C-clarinets in the orchestra. However, the sound of the A- and C-clarinet is so different in character from that of the B-clarinet that even if he had succeeded in the technical construction of such a normal clarinet to perfection, it would never have found acceptance with the better composers, as the tonal effects of the different clarinets produce effects which the composer endowed with a fine sense of tone will not want to do without." (Translated from German) 
At the end of the 18th century, the role of the clarinet began to change. Composers began to prefer using the lower ranges of the clarinet, and the earlier bright tones were replaced by warmer tones that were also suitable for expressing much deeper emotions. Perhaps the greatest pioneer in this field was Wolfgang Amadeus Mozart (1756-1791), who wrote his most famous work for clarinet in this way, the A major clarinet concerto. This also motivated later composers to use the entire range of the clarinet.
 Vienna Symphonic Library: Clarinet in Bb – History. [Online] available at: https://www.vsl.co.at/en/Clarinet_in_Bb/History (24.04.2019).
 Baermann, Carl: Vollständige Clarinett-Schule von dem ersten Anfang bis zur höchsten Ausbildung des Virtuosen. Offenbach am Main, 1864, S. 2.
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