Clarinets are usually made of dark wood, although good quality plastic clarinets are also common. Metal keys aid fingering, which can be quite fast on this very agile instrument.

When not being played, the clarinet is normally disassembled into several parts: the reed, ligature (which holds the reed on the mouthpiece), mouthpiece, barrel (or socket), upper body section, lower body section, and bell. The single reed, a thin, rectangular piece of a reed plant, must be replaced often.

The basic shape of a clarinet is a cylindrical tube open at one end. This strongly affects the harmonics of the instrument in two ways. (Please see Standing Waves and Wind Instruments for more information.) One is that the sound, particularly in the lower register, has unusually strong odd-numbered harmonics. This is what gives the clarinet its rich, complex timbre.

The other effect occurs when the player overblows to get a higher note with the same fingering. Since the next harmonic available is the third harmonic rather than the second, the clarinet overblows at the 12th rather than the octave. (Please see Standing Waves and Wind Instruments and Harmonic Series if you want to understand why.) This makes fingering more complicated for the clarinet than it is for instruments like the saxophone, which overblow at the octave. Twenty-four keys are needed to produce a smooth, in-tune chromatic scale through the entire range, and there is no uniform system of fingering. A single note can have many alternative fingerings which may be more or less useful in different situations.

The most common clarinet (the B flat) has such a large range, and its timbre varies so much over its range, that its different registers have been named. The low register, where the timbre is rich and dark, is called the chalumeau register. The higher clarinet register has a very clear, direct sound, and can be extremely expressive. The extreme upper register gets a shrill, piercing tone. In between the chalumeau and clarinet ranges (usually from G to B flat in the middle of the written staff), is the weaker throat register, where players can experience a difficult-to-negotiate "break" between the two registers. (This is partly caused by fingering difficulties, see above.)

The B flat clarinet is the most common modern instrument. It is a transposing instrument that sounds one whole step lower than written. Most band and orchestra clarinet sections also have one or more bass clarinets. The bass clarinet sounds an octave lower than the regular B flat clarinet - it is also a B flat transposing instrument - and is much bigger. It has an upturned bell (often silver), and, like the cello, must rest on a spike on the floor when it is played. The contrabass, or double bass clarinet is an octave lower than the bass clarinet and much bigger, standing six and a half feet high. Like the small E flat clarinet (which sounds a perfect fourth higher than the B flat), it is unusual, but can still be found. Many orchestral players have an A clarinet for playing in sharp keys, as well as a B flat instrument. Other clarinets, such as the C clarinet and the alto clarinet (about halfway between the B flat and the bass in size and range) have become rare.