Single reeds are used on the mouthpieces of clarinets and saxophones. They have a flat (back) side which fits against the mouthpiece and a top side which tapers to a thin tip. They are rectangular in shape except for the thin vibrating tip, which is curved to match the curve of the mouthpiece tip. Although all single reeds are shaped similarly, they vary in size to fit the appropriate mouthpiece.
The most obvious variation in reeds designed for the same instrument is a variation in thickness ("hardness" or "strength"), generally measured on a scale of 1 through 5 from softest to hardest. This is not a standardized scale and reed strengths vary between manufacturers. The thickness of the tip and heel and the profile in between also affect the sound and playability. Cane of different grades (density, stiffness), even if cut with the same profile, will also respond differently.