Crotal bells, also known as rumble bells, were used on horse-drawn vehicles before motorised vehicles were common.
They were orb shaped bells made of bronze with a slot cut down the side and a round iron ball placed inside, which made the bell ring when it moved. These bells were used to warn other horse drawn vehicle users (mostly on country roads) that another vehicle was approaching. They came in many sizes from a small 1 inch version to bells that were many inches across.
They were either hung on a small leather and iron harness bracket above the horse's collar on smaller vehicles. On larger vehicles, such as delivery wagons, they were driven into the wooden frame of the wagon.