Unlike any other balance, this one makes it possible to physically show that - 1 is less than 0. Weights placed on the outer pans represent negative integers and those placed in the inner pans represent positive integers. When you place a weight in each pan on one side of the center post, the balance remains level as ( - 1) + (+1) = 0. Great for small group work, the balance comes with two plastic pans, four plastic canisters, a set of plastic weights and a Teacher's Booklet. Some assembly required.