Plug Followers by Sparrow Lock Picks are a professionally designed they are an essential tool to have while taking a lock apart. The plug follower is placed in the housing of the lock while you remove the core. When you try removing the core without one of these tools don't be surprised if you see the springs and pins flying all over! This set comes with 4 plug follower sizes (.05, .55, .498, .398) and is made of anodised aluminnum. These are designed to fit in the majority of different size lock housing.