 Ricardo noticed that the bargaining power of laborers can never dip below the produce obtainable on the best available rent-free land, because whenever rent leaves them with less than they could get on that free land, they can simply move to the new location.The produce obtainable on the best available rent-free land is known as the margin of production.Since landlords have a monopoly over a given location, the only limiting factor for rent is the margin of production.