The least common denominator is the smallest number that has all of the factors of each fraction's denominator. In your example with the fractions and , we get from the first fraction that the LCD must have at least one factor of x and one factor of y. From the second fraction, we get that the LCD must have at least two factors of y. So the LCD has at least one factor of x and at least two factors of y. The smallest number that satisfies both of these requirements is .