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Complement vs. Supplement: Understanding the Difference

There are quite a few words that seem similar in meaning in English but that we cannot use interchangeably. Two examples are the words “complement” and “supplement.” Let’s try to understand the difference.

If you add something different that makes another item or idea better — or goes well with it — complement is the correct word to use. However, if you are adding something to an object or idea that could improve or increase its value, then you should use the word “supplement.”

Continue reading to learn more about the slight differences between these two words and how to be sure that you are using each correctly. Then, find out how the same words are used not only in writing and speaking but also in English grammar and mathematics.  

The Basic Difference between Complement and Supplement 

Quite a few words in English can act as different parts of speech — a slight shift in spelling makes a noun an adjective or a verb a noun.

It’s no wonder English can be confusing. Understanding some patterns and similarities in meaning will help you gain a better grasp of differences in usage, though.  

The word “supplement” can be both a noun and a verb, depending on its use. Remember that a noun is a person, place, thing, or idea. A verb is an action word. 

Here’s a quick example of “supplement” as a noun:

  1. The vitamins were a supplement to my already healthy diet.  

Now, let’s take a look at how it can be used as a verb:

  1. I supplement my diet with vitamins each day.

This time, the word supplement is an action word.  

The same is true of the word “complement” — it can be both a noun as well as a verb. 

While their meanings are similar, how these two words are used is a little bit different. To supplement something means that one thing is added to something else to improve, increase, or complete it (source). 

In other words, you are adding something extra — but similar in kind — to increase the value of what you already have.

“Complement,” on the other hand, is slightly different in that it means you are combining two different things or ideas to create a better whole. While the definitions of each word sound very close, you’ll notice a slight difference in usage and application.  

An easy way to double-check is to ask yourself, “Am I adding something new, different, and unrelated to create a better combination or whole?” If the answer is yes, then you are complementing what you already have.

If the answer is no, you will most likely need to use the word “supplement” instead.  

Below are a few examples that will help you understand the slight nuance in meaning between these two words.

1.     His boss decided to supplement his salary with a bonus for all of his hard work.

2.     His boss decided to complement his salary with added vacation time for all of his hard work.

If you look at the first sentence, the only idea or item we are working with is the subject’s salary. We are simply adding more to his current salary. For this reason, the word “supplement” is the correct word.

In the second sentence, however, we have two different ideas or items combined to create a better whole. The subject’s salary stayed the same, but he will also receive more vacation time, resulting in better compensation for his work.  

Additionally, in each of these sentences, both words act as verbs because the subject’s boss is performing an action.  

Let’s take a look at a couple more sentences to better understand the difference between complement and supplement.

1.     Ice cream complements cake wonderfully — they each taste better together.

2.     The writer supplemented her book with an additional chapter when new scientific evidence further supported her theory.  

Here, again, the question is whether or not we have two different, unrelated ideas that go together to create a better whole or whether we are adding something related to what is already there. 

Remember that something that complements something else goes well, one with the other. It does not add to or combine with it (source).

In the first sentence, we have two different items — cake and ice cream. Therefore, they complement each other because they are each two different items, and they go together very well to result in something even better.

In the second sentence, we have only one item — the writer’s book. The addition of a chapter is a supplement because, even though it is technically different from the first edition, it is the same subject matter and adds an additional element.

If we said that the writer was adding a free audio podcast with every purchase of her book, we would then use the word complement, as an audio podcast and her written book are two different things.  

There is one more word we should mention before we move on to ensure there is no confusion.  

The word “complement” is also a homophone. Homophones are two words that sound alike but that are not spelled the same and that have different meanings (source). Let’s take a look at “complement” versus “compliment” below.  

ComplementCompliment
Meaning: To make something better or more appealing by adding to it. Meaning: To express approval, admiration, or praise.
Example: Whipped cream complements strawberries, bringing out a natural sweetness and texture.Example: I gave my mom a compliment when I saw the new dress she was wearing.

Phonetically, both complement and compliment sound the same, /ˈkɒmpləmənt/ (source). But, if you look closely, the first (complement) is spelled with an e, while the second (compliment) is spelled with an “i.” 

Next, we’ll take a closer look at how both complement and supplement can also be used as adjectives with the addition of a simple suffix.

Complementary versus Supplementary 

Earlier, we said that compliment and supplement can be both nouns and verbs, depending on how they are used. We can also add the -ary suffix, changing complement to “complementary” and supplement to “supplementary.” 

Doing so changes each word from either a noun or a verb to an adjective. Remember that an adjective is a word that describes something or someone.    

Suffixes are used in quite a few different ways in English, and there are a lot of different letters and groups of letters that form suffixes. In essence, adding a suffix changes the spelling and sometimes the meaning of the word, as well as its part of speech. 

Below are a few examples of common suffixes:

  • -ful: forget becomes forgetful
  • -ment: state or govern becomes statement or government
  • -ee: employ becomes employee 
  • -ness: happy becomes happiness
  • -ship: friend becomes friendship
  • -ly: quick becomes quickly
  • -en: hard becomes harden

Here, we’re going to focus on the suffix -ary and its meaning as it applies to both complement and supplement.  

The -ary suffix means that something belongs to or is connected with something else.

In general, it will change a noun to an adjective. Adding the -ary suffix to complement or supplement does not change the meaning of either word but, rather, changes how you will use each in a sentence. 

Let’s take a look at some examples:

1.     The different colors in the scarf were complementary and created a perfect fall piece.

The word complementary, as it is used above, is an adjective that describes the colors in the scarf. We use complementary and not supplementary because, again, the colors are two different shades that work well together when combined. 

2.     My school provides supplementary programs for students who need some extra help.  

The word supplementary is also an adjective. Here, it describes the added programming that the school provides to students.

Supplementary is the correct word to use because the additional programs provided are related and add value to what is already available. Here’s another example: 

3.     I found the tea perfectly complementary to the lemon shortbread cookies served after lunch.  

Here, again, complementary is an adjective describing the tea. You’ll note that the two items, tea and cookies, are different. Therefore, complementary is the correct word choice.

4.     My second job provides supplementary income, but I enjoy it much more than my full-time work.  

Supplementary above, again, acts as an adjective, describing how the subject’s second job provides additional income. Since we are only talking about income, one idea, supplementary is the correct word choice.  

Recognizing the slight differences in the meanings of the words “complement” and “supplement” can be tricky, but, in time, you will more easily be able to determine which is the correct word to use. 

Supplementary versus Complementary in Mathematics

Supplementary and complementary are not only used in writing to describe words or phrases, they are also used in mathematics, so we’ll quickly cover how in this brief section.  

In math, these words describe the relationship between angles. A complementary angle is one in which two angles are added together to reach a sum of 90 degrees.

However, a supplementary angle refers to two angles that, when added together, reach a sum of 180 degrees (source).

A common supplementary angle is actually a straight line, while a common complementary angle is a right angle.

While the angles do not have to be next to or adjacent to each other, they do need to equal the exact sum, either 90 degrees or 180 degrees respectively, to be called either complementary or supplementary.  

You’ll notice that while the meanings of complementary and supplementary in mathematics are not the same as in general language, the root is somewhat similar. In both instances, we’re placing two things together.  

Complement and Supplement in English Grammar and Everyday Language

There are a lot of different ways these two unique, yet similar words are used in English. We’ve seen that they can act as different parts of speech, and they can also be found in mathematics. You’ll even find the word “complement” in your grammar lessons!

In grammar, a subject complement refers to a word or phrase that follows a linking verb — such as is, will, or am — and describes the subject more completely. It can also identify an object with more specificity.  

Here are a few simple sentences with subject complements:

1.     I am a mom.  

2.     The ice cream is vanilla.  

In the first sentence above, “a mom” is a subject complement because it further identifies the subject of the sentence, “I.” And “vanilla” is also a subject complement because it further specifies the flavor of the ice cream.

In each sentence, the subject complement is necessary in order to create a complete thought.

While subject complements are not the same as using the word “complement” as part of everyday speaking and writing, you’ll notice that the root meaning is, again, quite similar.

In both instances, you are adding something, whether another item or additional identification.

Complementary and supplementary are common words, and you’ll find them used often when speaking about particular subjects, such as food, fashion, business, and even genetics.

There are quite a few common food or drink combinations that complement one another, such as fruit and cheese, chocolate and mint, coffee and donuts, and peanut butter and jelly.  

In fashion, you’ll often hear how two pieces of clothing that work well together complement one another.

And in genetics, a gene that is considered complementary interacts with another gene in order to create a new, unique trait or characteristic (source). 

Similarly, you’ll often hear the word “supplementary” in reference to health, specifically vitamins and minerals, in education, and, again, even in genetics. The latter refers, this time, to genes that work together to contribute to a single characteristic.  

Final Thoughts

As you likely noticed by now, “complement” and “supplement,” while two distinct words with distinct definitions and uses are quite similar and have a multitude of applications. 

If you’d like to learn a few more words that seem similar but have different meanings, take a look at “Pronunciation versus Enunciation.” Before you know it, you’ll be well on your way to understanding these small nuances in language and word choice.