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.