Commas and Coordinating Conjunctions

I am going to start this discussion of commas by showing a simple pattern of comma use:

Some educators believe in using inductive methods in the classroom, and others maintain that a ‘top-down’ approach is more effective.

In this compound sentence, the comma separates the two independent clauses, indicating that we must read each one separately; the ‘and’ following a comma tells us that the next word (in this case, ‘others’) is the beginning of a new independent clause. The crucial issue here is the presence of a coordinating conjunction. A coordinating conjunction is strong enough to join two independent clauses with only a comma. There are seven coordinating conjunctions: and, but, so, or, nor, for, yet. Only those seven words can give us the compound-sentence-with-comma pattern found in the above example. I have listed them in a way that approximately reflects the frequency of their appearance in this capacity. Obviously, these are words with many other roles to play in our writing, but I am speaking here of their use as coordinating conjunctions. We use ‘and’ and ‘but’ all the time; we use ‘so’ and ‘or’ often; and we rarely (outside of literary writing) use ‘nor’, ‘for’, and ‘yet’. We can also write them in a different order so as to get a mnemonic: for, and, nor, but, or, yet, so. The resulting FANBOYS, despite being a somewhat silly word—I confess to having spent a fair amount of time rearranging those seven letters in an attempt to find a more dignified term—is a handy way to check if a sentence can be punctuated as a compound sentence.

Now that we understand this simple pattern of comma use in compound sentences, we can look at some related comma errors.

One such error is the comma splice: the placement of two independent clauses together with a comma and NO conjunction whatsoever. Here is an example:

The simulation of physical systems is a crucial part of scientific discovery, experience shows that conducting this simulation precisely and efficiently is essential.

This type of error—which is relatively rare—can be easily fixed in a number of ways, including the simple addition of ‘and’ before ‘experience’:

The simulation of physical systems is a crucial part of scientific discovery, and experience shows that conducting this simulation precisely and efficiently is essential.

What if we had attempted to correct this sentence by replacing the comma with an ‘and’? Note the ambiguity of a compound sentence without the comma:

The simulation of physical systems is a crucial part of scientific discovery and experience shows that conducting this simulation precisely and efficiently is essential.

Here it would be easy to read ‘scientific discovery and experience’ as a single phrase and thus miss the true structure of the sentence. Remembering that we always need a comma before a coordinating conjunction in a compound sentence will save us from this potential ambiguity.

Lastly, I would like to look at a related comma error that is very common: the practice of using commas to separate independent sentences joined by conjunctive adverbs (e.g., however, therefore, accordingly, finally, instead, nevertheless, specifically, thus) or transitional expressions (e.g., equally important, for example, in fact, on the contrary, on the other hand). Compare these two sentences:

Low levels of ROS are used in redox signalling reactions that are essential for cellular homeostasis, but high levels of ROS initiate an intracellular response that leads to oxidative stress.

Low levels of ROS are used in redox signalling reactions that are essential for cellular homeostasis, however high levels of ROS initiate an intracellular response that leads to oxidative stress.

The first sentence is correct, but the second one is not. Here is a corrected version of the second sentence:

Low levels of ROS are used in redox signalling reactions that are essential for cellular homeostasis; however, high levels of ROS initiate an intracellular response that leads to oxidative stress.*

In order to understand this distinction, we have to see the difference between ‘but’ and ‘however’. This distinction can seem opaque since the two terms have a similar meaning. However, we now know that ‘but’ is a coordinating conjunction, which means that it is strong enough to connect two independent clauses. Other words or phrases—such as conjunctive adverbs or transitional expressions—will fail at this particular task. Knowing that only coordinating conjunctions work in this sentence pattern and being able to remember the seven coordinating conjunctions (hence the value of the mnemonic, however lame) will allow you to avoid this particular type of comma error.

Let’s look at a final example of this type of error:

The design process of a UAV starts with a pre-specified mission agenda, consequently, following conventional design methods will lead to an airplane that is only suited to achieving the primary mission.

If you find that sort of sentence in your own writing, you can do one of three things:

1. Use a semicolon.

The design process of a UAV starts with a pre-specified mission agenda; consequently, following conventional design methods will lead to an airplane that is only suited to achieving the primary mission.

This is by far the easiest and most effective solution. The semicolon probably best reflects the close relationship that the original sentence was trying to create.

2. Use a coordinating conjunction to preserve the original sentence pattern.

The design process of a UAV starts with a pre-specified mission agenda, and following conventional design methods will lead to an airplane that is only suited to achieving the primary mission.

This solution is grammatically correct but may not retain the original meaning. In this case, for instance, replacing ‘consequently’ with ‘and’ is a clear alteration of the original meaning.

3. Use a period.

The design process of a UAV starts with a pre-specified mission agenda. Consequently, following conventional design methods will lead to an airplane that is only suited to achieving the primary mission.

This is probably the least effective solution. Although it is grammatically correct, it does not express the close relationship between the two parts of the sentence. In general, replacing these sort of commas with periods can lead to unnecessarily short sentences and thus choppiness.

In sum, understanding the proper use of coordinating conjunctions will allow you to construct correct compound sentences and will allow you to avoid problems with comma use preceding conjunctive adverbs and transitional expressions.

The next comma post will look at how commas are used as a way to cope with long sentences.

* Comma use after introductory elements will be discussed in a future post.