Definition
Unlike denominators
Fractions have unlike denominators when their denominator numbers are different.
What it means
Thirds and fourths are different-sized pieces of the same whole. You cannot add their counts as though they were the same kind of piece. Rename both fractions in a shared unit—such as twelfths—then combine.
Remember it
Different bottom numbers mean different named pieces.
Example
1/3 + 1/4 becomes 4/12 + 3/12 = 7/12. Twelfths give both fractions the same unit.
Watch for
Different denominators do not stop comparison; benchmarks or models may compare them without first making a common denominator.
Common mix-up
Adding 1/3 + 1/4 as 2/7 mixes piece counts with piece names and is not valid.
Exact meaning
Unlike denominators are different denominator values in two or more fractions, so the fractions name different fractional units. For addition or subtraction, they are normally rewritten as equivalent fractions with a common denominator before numerators are combined.