Complement / Compliment

These two words, which effectively are pronounced the same, are often confused.

However, their meanings are different. Both can be used as a noun and a verb; also as an adjective in the form complementary / complimentary. 

Complement

This word means to compete or to fill. Examples of its use are:

As a noun:

No more passengers were allowed on the ship because the number of people on board had reached the complement.

As a verb:

The desert complemented the enjoyable three-course meal.

In grammar terminology, the word complement is used to define part of sentence which has an intransitive verb; for example, The town is busy. In this sentence, town the is the subject and busy is the complement as it completes the sentence.

(Note: Where the verb is transitive, it has as object rather than a complement; for example: Today I cleaned the car. In this sentence, car is the object)

Compliment

This word means to show appreciation, to praise or to honour.

Examples: 

As a noun:

I enjoyed the meal. Please give my compliments to the chef. 

As a verb:

The professor complimented the students on the high quality of their essays. 

Complementary / Complimentary

These are the adjectives derived from complement/compliment.

Examples:

The additional lecture is complementary to the course.

This means that the course is completed by the additional lecture.

The customers made complimentary comments about the high standard of service in the shop.

This means that the customs appreciated the service given to them. 

The word complimentary also has another meaning which means to provide something free of charge.

Examples:

It is usual for the author of a book to give a complimentary copy to everyone who is mentioned in the acknowledgements.

Twenty people, who otherwise could not afford to buy a ticket, were give a complimentary ticket for the concert.

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