Woodwind instruments have changed a lot since the old times. Flutes, oboes and bassoons hardly had any keys (the metal keys which help to cover the holes). The keys were added in the 18th century and it made it much easier to play difficult music with lots of sharps and flats. Clarinets were not invented until the late 18th century, but even they have developed a lot since Mozart’s time (1756-1791).