lawless523 asks, “When do you use ‘complimentary’ versus ‘complementary’?”
The difference between compliment and complement was expertly covered by
traycer_ in this answer, where we learn that to compliment means “to pay a compliment; to show kindness or regard for by a gift or other favor” and to complement means “to complete” or “to make perfect.”
In that case, complimentary would mean “of the nature of, conveying, or expressing a compliment”; “given free as a gift or courtesy”; or “something given or supplied without charge.”
“Those were the best spring rolls you’ve ever made, Hakkai!” said Goku.
“Yeah, they were pretty awesome,” Gojyo agreed. Sanzo just sipped his beer and grunted.
Hakkai raised an eyebrow. “Goodness, you’re all so complimentary tonight-especially you, Sanzo.”
“Gah! I think the complimentary soap at the last inn gave me a rash!” Gojyo said, scratching furiously.
Complementary means “completing” or “forming a complement” -in other words, two or more things can combine together to either emphasize or enhance each other’s qualities.
Sometimes Hakkai would look up at the night sky and think about how he and Gojyo were complementary, like the dark and light sides of the moon. And then he’d smile, knowing Gojyo would roll his eyes and say what a horrible cliché that was, even if it was true.
While most of the monks didn’t think orange and blue looked good together, Koumyou Sanzo found the two colors to be beautifully complementary-which is why he always flew orange paper airplanes on clear days.
Sanzo was busy drowning his ramen in mayonnaise while Gojyo and Goku looked on in horror. Hakkai never understood how Sanzo thought the two flavors could possibly be complementary: bland and blander didn’t enhance one another, unlike pot stickers with just the right amount of ginger to bring out the flavor of the pork filling.
So when you want to describe something as being free, a gift, flattery, or a kindness, use complimentary. If it’s a question of completing, perfecting, or enhancing something, though, use complementary.
Sources:
“Complementary” at Dictionary.com
“Complimentary” at Dictionary.com