The reed is a thin piece of arundo donax that is attached to the mouthpiece to produce the clarinet's sound. By blowing air past the reed, the reed vibrates. This does not produce the sound however, but rather this vibrates the column of air within the clarinet, which produces the sound.
In concert playing, natural cane reeds are often used to produce a better sound, while in marching bands many people choose to use synthetic reeds which have a poorer tone, but are more durable for marching.
It is well worth experimenting with different makes and strengths of reed, as they can have a great impact on playing.
Reeds come in different strengths for different levels of experience; starting at 1, the tone is easy to produce, but the high register is hard to reach. Strengths go up by quarters with harder reeds being size 4-5. With the hardest reeds the reed offers more resistance but it is easier to reach the higher notes, and a better tone can be reached. As strength increases the thickness of the reed increases too.
In general, it is recommended that beginners start with a soft reed. Clarinettists usually find that they need to progress to harder reeds once their embouchure has developed. However, this does not mean that playing on a strength 4 reed makes a clarinettist very good; in fact it is unnecessary and even inadvisable for many people to play on reeds of that strength.
The strength of a reed can be altered to make it softer or harder. Reed rush or a knife can be used to scrape a little of the cane away from the centre of the reed, known as the heart, which has the effect of making the reed softer. Shortening the reed by shaving cane off the tip makes the reed harder, and a tool designed for this purpose, called a reed clipper is usually used for this. However, if the clarinettist is consistently altering reeds in the same way it may be worth considering using a different strength.
Many factors affect the sound a reed will produce. Two reeds from the same box will often produce different results, and one reed will play differently from one day to the next. Factors affecting reeds include humidity and temperature.
The storage of reeds is important to minimise the impact of these factors on the reed and thus prolong its playing life. For example, the plastic covers that reeds are sold in are not the best way to store reeds. Much better would be a specially designed box or case (usually available from woodwind retailers), which helps to regulate the temperature and humidity the reeds are exposed to.
It is also important to avoid chips and other damage to the reed. Mouthpiece caps should be over the mouthpiece when the clarinet is not in use.