it just starts at 0 in the middle and counts upward going both left and right...it helps you center things easily when you layer projects...fold each in half, mark the middle and then line up all the middle marks on the '0'....each edge should end up on the same left and right number and you will know you are all set... i made my own inexpensively by using cheap tape measures and sewing them onto the leaders of my quilt frame...