Modern serpentine belt systems utilize the same type of pulley on most accessories, with the only difference being the groove ...

Jockey pulleys are used with flat belts to increase the arc of contact between the belt and the pulley. This helps to prevent slippage and improve power transmission.

Unfortunately for me, because the idler pulley mounting bolt was loose ... one of the largest producers of drive belts in the world, low belt tension is the main reason for squealing belts.