The bourdon is the heaviest of the bells that belong to a musical instrument, especially a chime or a carillon, and produces its lowest tone.
As an example, the largest bell of a carillon of 64 bells, the sixth largest bell hanging in the world, in the Southern Illinois town of Centralia, is identified as the 'bourdon.' It weighs 11,000 pounds and is tuned to G. The Centralia Carillon was a single-handed project of the late William V. Joy, publisher of the Centralia Evening Sentinel, and was completed in 1986.
The biggest bell serving as bourdon of any carillon is the low C bell at Riverside Church, New York City. Cast in 1929 as part of the Rockefeller Carillon, it weighs 41,000 pounds and measures 10 feet, 2 inches across. This is also the largest tuned bell ever cast.
Although carillons are by definition chromatic, the next bell up from the bourdon is traditionally a whole tone higher in pitch, leaving a semitone out of the instrument.