A reed is one of the basic elements for creating a tone on the clarinet. If the reeds are too soft or too hard, they make aliquots (wrong notes), “ugly” tones, or cause difficulties in producing the tones. Such reeds create big problems and make it hard to play.
The good reed is one that produces a clear and pleasant tone. If the reed does not waste too much air, it means that its hardness is adequate; playing with the tongue is easy and clear and without failure.
The reed should not be chosen at the end of the day, when tiredness of lips is present (muscles and nerves), but fifteen minutes after playin.
Adapting to the new reed and adjusting it to us usually takes 3 to 4 days.
It is best to set aside 3 to 4 reeds of similar quality and use them interchangeably when practicing – for twenty minutes on each. You should mark with a pen each reed on the back with some sign so that you could distinguish them. In 3 to 4 days it will show which reed is the best. Of course, for live performance choose the best ones.
The reed should never be chosen on the day of perfomance. The new reed is not reliable because it has not passed all test-controls described above. It can often fail and significantly reduce the quality of your public appearance, and thereby spoil all the efforts invested to master a piece.
Taken from my book