Here’s a duo of easily confused words separated by only one vowel. But it’s a very important vowel. The word complementary with an “e” describes things that combine to enhance each other’s qualities. Complimentary with an “i” describes things that convey praise or things that are free, like “compliments of the house.”

(Here’s a simple mnemonic to help you remember: If I get something free, it’s complimentary.)
Example :
1. Those two product lines are complementary.
2. He was very complimentary about my performance at the conference.