The smallest range of bells that can constitute a carillon is two chromatic octaves, or 23 bells. (Historically, the lowest C-sharp and E-flat were often not installed for reasons of space and expense.) An instrument with less than 23 bells is a chime. A chime is a uniquely American instrument that usually consists of one to one and a half diatonic octaves.
Carillons range in size from two to greater than six octaves, or from a minimum of 23 bells to as many as 77. A range of four to four and one-half octaves (47 to 56 bells) is most desirable, since almost all carillon music can be played on such an instrument. (By comparison, a piano has 88 notes, while an organ keyboard has 61.) Most contemporary carillon music, and much historic music, is written for carillons with a range of four or more octaves. The available repertoire for smaller instruments is more limited.
The pitch of the bourdon bell of a carillon is often dictated by non-musical considerations, since both pitch and cost are directly related to the size of the bourdon. The largest carillon bell in the world is the bourdon of the Rockefeller carillon in Riverside Church, New York City. It sounds the note C, weighs more than 40,000 pounds (20 tons) and exceeds 10 feet in diameter. A bell sounding its octave (low C in the four-octave range) weighs approximately 5,000 pounds (2.5 tons) with a diameter of about 5 feet. A bell sounding the next octave (middle C) weighs approximately 580 pounds (a quarter-ton) and has a diameter of approximately 30 inches. The size and weight-bearing capacity of the bell chamber also has a direct bearing on the size of a carillon.
Since a carillon is seldom played with another instrument, the bourdon may be any pitch deemed suitable for the installation and funds available. Regardless of the actual pitch of the instrument, it is common for the keyboard to be laid out based on the key of C, to simplify the writing and playing of music for it. Thus many carillons are transposing instruments. This is more likely to be true of small or older instruments; modern instruments are more likely to be in concert pitch.
The carillon evolved as a folk instrument, played automatically as a clock chime, and by a performer for market days and holidays. Surviving music from the first "golden age" is mostly arrangements of folk tunes, dance pieces and popular music of the period, although there are some original compositions for carillon. Like piano music, carillon music is written on two staves: music on the treble or upper staff is played with the hands, while that on the bass or bottom staff is played with the feet.
Early in this century, the ability to arrange music from other sources and to improvise on the carillon was a necessary skill for the carillonneur. The growth of the carillon art in modern times has resulted in a rich repertoire with a wide variety of regional styles.
Care must be taken when composing or transcribing music for the carillon. One feature that sets the carillon off from other instruments is the fact that once a bell is struck, it continues to ring until the vibrations die out naturally. All of the musical expression of which the carillon is capable is controlled by how the performer strikes the bell. There is no way to stop or alter the sound of a struck bell. Dampers are ineffective because they just deaden the sound without stopping it, making it unmusical.
Another feature of carillon music derives from a prominent minor third in the bell's overtone structure, requiring care in formation of chords. Most musical instruments produce sounds with major thirds in their overtone structure.
Lastly, the rich tonal structure of a bell means that sounding a large number of bells together is unnecessary. A rich sound can be obtained from just a few bells.