First I need to say that the common multiple is when two or more numbers meet up at the same point like 5 and 3 meeting up at 15. The common denominator works almost exactly the same way. It involve two fractions' denominators meeting up like 1/5 and 1/3 meeting up at ?/15 with ? being what the numerator would be (like 1/5 becoming 3/15 because 5 x 3= 15 and you must do so to the numerator so 1 x 3=3. 3/15). So they are alike in the way that they both involve two or more numbers' multiples meeting up, and the common multiple is basically used to find the common denominator.