Inside is usually a tongue or clapper that makes a ringing sound when it strikes the inside surface of the bell. Bells come in every size, from thimble sized to the biggest bell in the world, Russia's Tsar-Kolokol (which is broken, incidentally). Bells come in different shapes, too, from those with narrow and long shapes to bells with a rectangular opening, like a cow bell. Bell ringers are pressed into service by churches, schools and other bodies to ring one or more bells. Sets of church bells are often rung by several bell ringers.
Use of bells goes back almost as far as recorded history. They are described in the Old Testament as being attached to a high priest's ephod, which was a special garment worn by high priests and kings. The book of Zechariah describes bells attached to the bridles or belts on the necks of horses that were trained for war. The idea was to get the horses accustomed to noise. Large bells were used to call soldiers to battle and Christians to church, and is associated with Christian churches more than other ones. One bell was rung when a person in the parish died, followed by two pulls to indicate if the decased was a woman, or three pulls if it was a man. Bells in the Christian church have been used as part of the mass, and were used before clocks were common to let people know the time of day. Bell music was first attempted in medieval times with rows of small bells rung with hammers.
Bells were one of the many types of musical instruments used during Medieval times. But they weren't just used for music; they had other practical applications, too. One use was by falconers, who attached small bells to their falcon's feet. That way the bird could be more easily located by its owner. Bells music were surrounded by superstition in medieval times and were actually baptized so that they could ward off evil spirits, particularly around doors where spirits were thought to be waiting for a chance to go inside. Drawings from the Middle Ages showed hand bells hanging from rods with instructions for the sung masses. Bells were often used to double upon the tenor line, which was the one that carried the melody. Later in the middle ages, the pipe organ gradually took over and strings, bells, harp, and wind instruments faded into the background, though tower bells continued to be rung and continue into modern times in some places.