Karotkin et al discovered that sets of weighted majority rules possess a property of essential order.However,they didn't explain the reason of that. In this paper,we proposed the concept of decision-rule chain and distance function of decision rules,and pointed out that when a set of decision rules compose a decision-rule chain they possess the property of essential order.Therefore the property of essential order is explained.And we can examine whether a set of decision rules compose a decision-rule chain b...