The Greatest Common Divisor (GCD) is the largest number that divides two integers evenly, and the Least Common Denominator (LCD) is the smallest number that both denominators divide into.
To simplify 12/18, find GCD(12, 18) = 6, then divide to get 2/3.
To add 1/4 + 1/6, find the LCD, which equals the least common multiple of 4 and 6: since LCM(4, 6) = 4 × 6 / GCD(4, 6) = 24 / 2 = 12, the LCD is 12.
As Wolfram MathWorld explains, the GCD and LCM are linked by the identity a × b = GCD(a, b) × LCM(a, b), which our calculator uses to keep denominators as small as possible.