This is the piece that is mounted to the bowling ball. The Vise Inserts Ball-It and Ball Expander are used in conjunction with a Vise IT slug. These two pieces are mounted inside the thumb hole of your bowling ball. The Ball-It is used to hold your IT thumb slug in place inside the thumb hole. The Ball-IT fits all three size of Vise IT thumb slugs - 1 1/8", 1 1/4", 1 3/8"