The number on the reed indicates its stiffness, or "strength". Larger numbers indicate stronger (thicker) reeds. Most instructors will start students out on #2 reeds (which are relatively soft) and gradually increase the strength as the student develops. The strength of the reed determines the tone that is produced. Softer reeds are easier to play, but have poorer tone. Stronger reeds produce a darker tone, but also require better breath support and embouchure strength.

If a reed is too soft for a player, it will tend to be buzzy or bright. The upper register may be flat, or may not speak at all if the player pinches the reed shut against the mouthpiece.

If the reed is too stiff, the articulation will suffer and the player will lose flexibility in the intonation. It will also be difficult to play softly or in the lower register.

If you are unsure about which size of reed to get, consult with your instructor. They will recommend a size that will suit your style and ability.