What is the most commonly shaped base for pin curls?

The arc base is the most commonly shaped base for pin curls because it allows for flexibility in the curl and can create a more natural wave. The arc shape follows the head's contour, which helps the pin curl to sit closer to the scalp and provides better support for the curl. This configuration also allows hair to blend more seamlessly with the overall hairstyle, creating a more polished look.

Additionally, the arc base is easy to create and manipulate, making it favorable for various styling techniques. The design of the arc base helps to produce softer, more consistent curls, which is often the desired outcome in hair styling. This makes the arc base the go-to choice for many stylists when working with pin curls.