Conjunctions are the words that join sentences, phrases, and other words together. Conjunctions are divided into several categories, all of which follow different rules. We will discuss coordinating conjunctions, adverbial conjunctions, correlative conjunctions, and subordinating conjunctions.

Coordinating Conjunctions

The most common conjunctions are and, or, and but. These are all coordinating conjunctions. Coordinating conjunctions are conjunctions that join, or coordinate, two or more equivalent items (such as words, phrases, or sentences). The mnemonic acronym FANBOYS can be used to remember the most common coordinating conjunctions: for, and, nor, but, or, yet, and so. Here are some examples of these used in sentences:

• Nuclear-powered artificial hearts proved to be complicated, bulky, and expensive.
• Any external injury or internal injury put patients at risk of uncontrolled bleeding because the small clots that formed throughout the circulatory system used up so much of the clotting factor.
• The current from the storage batteries can power lights, but the current for appliances must be modified within an inverter.

As you can see from the examples above, a comma only appears before these conjunctions sometimes. So how can you tell if you need a comma or not? There are three general rules to help you decide.

Rule 1: Joining Two Complete Ideas

Let’s look back at one of our example sentences:

The current from the storage batteries can power lights, but the current for appliances must be modified within an inverter.

There are two complete ideas in this sentence (a complete idea has both a subject and a verb). Because each of these ideas could stand alone as a sentence, the coordinating conjunction that joins them must be preceded by a comma. Otherwise you’ll have a run-on sentence.

Rule 2: Joining Two Similar Items

So what if there’s only one complete idea, but two subjects or two verbs?

1. Any external injury or internal injury put patients at risk of uncontrolled bleeding because the small clots that formed throughout the circulatory system used up so much of the clotting factor.
2. In the 1960s, artificial heart devices did not fit well and tended to obstruct the flow of venous blood into the right atrium.

The first sentence has two subjects: external injury and internal injury. They are joined with the conjunction and. The second sentence has two verbs: did not fit well and tended to obstruct. They are joined with the conjunction and. In both sentences, we don’t need any additional punctuation.

Rule 3: Joining Three or More Similar Items

What do you do if there are three or more items?

• Anna loves to run, David loves to hike, and Luz loves to dance.
• Fishing, hunting, and gathering were once the only ways for people do get food.
• Emanuel has a very careful schedule planned for tomorrow. He needs to work, study, exercise, eat, and clean.

As you can see in the examples above, there is a comma after each item, including the item just prior to the conjunction. There is a little bit of contention about this, but overall, most styles prefer to keep the additional comma (also called the serial comma). We discuss the serial comma in more depth in Text: Commas.

Adverbial Conjunctions

Adverbial conjunctions link two separate thoughts or sentences. When used to separate thoughts, as in the example below, a comma is required on either side of the conjunction. When used to separate complete ideas (items with both a subject and a verb), a semi-colon is required before the conjunction and a comma after.

• The first artificial hearts were made of smooth silicone rubber, which apparently caused excessive clotting and, therefore, uncontrolled bleeding.
• The Kedeco produces 1200 watts in 17 mph winds using a 16-foot rotor; on the other hand, the Dunlite produces 2000 watts in 25 mph winds.

Adverbial conjunctions include the following words; however, it is important to note that this is by no means a complete list.

Correlative Conjunctions

Correlative conjunctions are word pairs that work together to join words and groups of words of equal weight in a sentence. This video will define this types of conjunction before it goes through five of the most common correlative conjunctions:

The table below shows some examples of correlative conjunctions being used in a sentence:

Subordinating Conjunctions

Subordinating conjunctions, are conjunctions that join an independent clause and a dependent clause. Here are some examples of subordinating conjunctions:

• The heart undergoes two cardiac cycle periods: diastole, when blood enters the ventricles, and systole, when the ventricles contract and blood is pumped out of the heart.
• Whenever an electron acquires enough energy to leave its orbit, the atom is positively charged.
• She did the favor so that he would owe her one.

Let’s take a moment to look back at the previous examples. Can you see the pattern in comma usage? The commas aren’t dependent on the presence subordinating conjunctions—they’re dependent on the placement of clauses they’re in. Let’s revisit a couple examples and see if we can figure out the exact rules:

• The heart undergoes two cardiac cycle periods: diastole, when blood enters the ventricles, and systole, when the ventricles contract and blood is pumped out of the heart.
  • These clauses are both extra information: information that is good to know, but not necessary for the meaning of the sentence. This means they need commas on either side.
• Whenever an electron acquires enough energy to leave its orbit, the atom is positively charged.
  • In this sentence, the dependent clause comes before an independent clause. This means it should be followed by a comma.
• She did the favor so that he would owe her one.
  • In this sentence, the independent clause comes before an dependent clause. This means no comma is required.

The most common subordinating conjunctions in the English language are shown in the table below: