What Is LCD (Least Common Denominator)?
The least common denominator, or LCD, is the smallest number that works as a common denominator for a set of fractions. It's really just the LCM applied to the fractions' denominators. Once every fraction shares the LCD, they can be added or subtracted directly.
Example
The LCD of 1/4 and 1/6 is 12, since 12 is the smallest number both 4 and 6 divide into evenly.