Bernina belts come in two basic sizes, primary short and secondary long. Some of the very old models have a single belt.
The shorter belts fit onto the motor and then the jockey wheel. The longer belts fit from the balance, or hand, wheel and then to the jockey wheel.
In most cases Bernina belts will last a very long time, they are well made and do the job without much complaint. They will suffer if they are allowed to become slack as this will often create a ‘flat spot’ on the belt as the motor pulley or even the balance wheel spins before the belt grips again. Another reason to regularly check your Bernina or have it serviced by a Bernina agent.