This equation implies two things. First buying one more unit of good x implies buying formula_7 less units of good y. So, formula_7 is the "relative" price of a unit of x as to the number of units given up in y. Second, if the price of x falls for a fixed formula_1 and fixed formula_10 then its relative price falls. The usual hypothesis, the law of demand, is that the quantity demanded of x would increase at the lower price. The analysis can be generalized to more than two goods.